Mitigating DMARC False Negatives with Third-Party Senders and Subdomain Delegation

11 August 2026 · DMARC Engine · 35 min read

The False Negative Conundrum: A Real-World Scenario

At DMARC Engine, we often encounter customers who are struggling to mitigate false negatives, particularly when dealing with third-party senders and subdomain delegation. A false negative occurs when a legitimate email is incorrectly flagged as spam or blocked due to a DMARC policy misconfiguration. This can lead to a significant loss of reputation and revenue for the organisation. To illustrate the complexity of this issue, let us consider a real-world scenario.

Suppose we have a customer, a large e-commerce company called example.co.uk, which uses a third-party email service provider, marketing.example.net, to send promotional emails to their subscribers. The company has implemented DMARC with a policy of quarantine, which means that any email that fails DMARC validation will be quarantined by the receiving email server. However, the third-party email service provider is not configured to send emails with a valid DMARC signature, resulting in a high rate of false negatives.

To make matters worse, example.co.uk has also delegated subdomains to other third-party providers, such as feedback.example.co.uk for customer feedback and support.example.co.uk for customer support. These subdomains are not properly configured for DMARC, which further exacerbates the false negative problem.

In our experience, the root cause of false negatives often lies in the misconfiguration of SPF or DKIM records. For instance, the SPF record for example.co.uk may not include the IP addresses of the third-party email service provider, resulting in emails being flagged as spam.

example.co.uk. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.2 -all"

In this example, the SPF record only includes the IP addresses of the company's own email servers, but not those of the third-party provider. To fix this issue, we would need to update the SPF record to include the IP addresses of the third-party provider.

example.co.uk. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.2 include:marketing.example.net -all"

Similarly, the DKIM record for example.co.uk may not be properly configured, resulting in emails being blocked by the receiving email server.

default._domainkey.example.co.uk. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC+yt4abHj6tqp6gM4kTkjRn9jGc9x7d0xH4R0f9Rg4c4b2a8oV9x3g"

In this example, the DKIM record is using a default selector, which may not be compatible with the third-party email service provider. To fix this issue, we would need to update the DKIM record to use a custom selector that is compatible with the third-party provider.

marketing._domainkey.example.co.uk. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC+yt4abHj6tqp6gM4kTkjRn9jGc9x7d0xH4R0f9Rg4c4b2a8oV9x3g"

In a hosted or managed setup, such as the one offered by DMARC Engine, these issues can be easily identified and resolved through the use of automated tools and expert analysis. Our platform provides a comprehensive view of the customer's DMARC configuration, including SPF and DKIM records, and alerts them to any potential issues that may be causing false negatives.

However, in a self-managed setup, it can be challenging for organisations to identify and resolve these issues on their own. This is particularly true for large organisations with complex email infrastructures and multiple third-party providers. In such cases, it is essential to have a thorough understanding of DMARC, SPF, and DKIM, as well as the ability to analyse aggregate reports and identify potential issues.

To mitigate false negatives, organisations should prioritise the proper configuration of SPF and DKIM records, as well as the use of subdomain delegation. They should also ensure that all third-party email service providers are configured to send emails with valid DMARC signatures. Also, organisations should regularly monitor their aggregate reports to identify potential issues and take corrective action to prevent false negatives.

By taking a proactive approach to DMARC configuration and management, organisations can reduce the risk of false negatives and ensure that their legitimate emails are delivered to their intended recipients. In the next section, we will delve deeper into the impact of third-party senders on DMARC and explore strategies for mitigating false negatives in these scenarios.

Understanding the Impact of Third-Party Senders on DMARC

The presence of third-party senders can significantly complicate DMARC implementation, as these senders often require the ability to send email on behalf of a domain, which can lead to DMARC false negatives if not properly configured. A false negative, in this context, occurs when a legitimate email sent by a third-party service is incorrectly flagged as spam or blocked due to DMARC policy. This can happen when the third-party sender's IP address is not included in the domain's SPF record, or if the DKIM signature does not align with the domain's DMARC policy.

For instance, consider a company like Example Ltd, which uses a third-party marketing service, MarketSender, to send newsletters to its customers. MarketSender uses its own servers to send these emails, but the emails are sent from the example.com domain. If Example Ltd has implemented DMARC with a policy of quarantine, and MarketSender's IP address is not included in Example Ltd's SPF record, there is a high likelihood that these marketing emails will be quarantined by receiving mail servers, resulting in a false negative.

To mitigate this issue, it is essential to properly configure SPF and DKIM for third-party senders. One approach is to include the third-party sender's IP address in the domain's SPF record. However, this can become cumbersome if the domain works with multiple third-party senders, each with its own set of IP addresses.

# Example SPF record for example.com including MarketSender's IP address
example.com. IN TXT "v=spf1 include:_spf.marketsender.com -all"

In a hosted or managed setup, such as the one provided by DMARC Engine, this process can be simplified through automated tools that help manage SPF and DKIM configurations for third-party senders. For example, DMARC Engine's platform allows users to easily add or remove third-party senders from their SPF records, and also provides guidance on how to properly configure DKIM for these senders.

Another critical aspect to consider is the alignment of DKIM signatures with the domain's DMARC policy. DKIM allows a domain to take responsibility for a message by adding a digital signature to the headers of the message. For DMARC, it is crucial that the DKIM signature aligns with the domain's DMARC policy, meaning the signature must be issued by a domain that is a subdomain of or the same as the domain in the From header of the email.

# Example DKIM record for example.com
default._domainkey.example.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC+ycpQMMj6qRqjijM0H19z3mG8Q6NYB5HUPKkR0l4e0N1L1ftKs+6k4M4mT9g9H4eDZToH9r1Q9urh5V6u3blJzRWEOr+4LbXz0lT9K0Z4Vz38yD6qBQ9xGf6S5odF0+5K8YdQYp4n9ae6WZb5I2O3q5JDPxL3/k18wIDAQAB"

In the case of third-party senders, ensuring DKIM alignment can be challenging, especially if the sender does not support DKIM signing or if the signing domain does not match the From domain. To address this, some third-party senders offer the option to use a custom DKIM selector, allowing the domain owner to specify the DKIM signing domain. This can help ensure DKIM alignment and prevent false negatives.

The decision to use a custom DKIM selector versus relying on the third-party sender's default DKIM setup involves trade-offs. Using a custom selector provides more control over DKIM alignment but requires more technical setup and management. On the other hand, relying on the default setup might be simpler but could lead to DKIM alignment issues if not properly configured.

In practice, managing third-party senders in a DMARC implementation requires careful planning, ongoing monitoring, and sometimes, compromises. For domains with a large number of third-party senders, it might be necessary to use a combination of SPF and DKIM, along with regular review of DMARC aggregate reports to identify and address any issues related to these senders. DMARC Engine's experience with managing DMARC for various customers has shown that proactive management of third-party senders is key to minimising false negatives and ensuring the effectiveness of DMARC protection.

Ultimately, the goal is to find a balance that optimises email deliverability while maintaining the security benefits of DMARC. By understanding the impact of third-party senders on DMARC and taking a proactive approach to configuring SPF and DKIM for these senders, domains can significantly reduce the risk of false negatives and enhance their overall email security posture.

Subdomain Delegation: A Double-Edged Sword for DMARC

Subdomain delegation is a crucial aspect of DMARC implementation, allowing organisations to delegate email sending to subdomains while maintaining control over their primary domain's email authentication. However, this delegation can be a double-edged sword, as it introduces additional complexity and potential risks. In our experience managing DMARC for customers, we have seen that subdomain delegation, if not properly configured, can lead to false negatives, which can have significant consequences for email deliverability.

One of the primary challenges with subdomain delegation is ensuring that the subdomain's DMARC record is correctly aligned with the primary domain's record. For instance, if a company has a primary domain example.com and a subdomain mail.example.com, the DMARC record for mail.example.com should be aligned with the record for example.com. This alignment is critical to prevent false negatives, which can occur when a mail receiver checks the DMARC record for the subdomain and finds it to be non-aligned with the primary domain's record.

To illustrate this point, consider the following DMARC record snippet for example.com:

_dmarc.example.com. IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:dmarc@example.com; ruf=mailto:dmarc@example.com; fo=1"

In this example, the DMARC record for example.com specifies a rejection policy (p=reject) and a reporting address (rua=mailto:dmarc@example.com). If the subdomain mail.example.com has a DMARC record that is not aligned with this record, it may cause false negatives. For example, if the DMARC record for mail.example.com is:

_dmarc.mail.example.com. IN TXT "v=DMARC1; p=none; pct=100; rua=mailto:dmarc@mail.example.com; ruf=mailto:dmarc@mail.example.com; fo=1"

In this case, the subdomain's DMARC record has a different policy (p=none) and reporting address (rua=mailto:dmarc@mail.example.com) than the primary domain's record. This non-alignment can cause mail receivers to reject emails sent from the subdomain, resulting in false negatives.

To mitigate this risk, it is essential to ensure that the subdomain's DMARC record is correctly aligned with the primary domain's record. One way to achieve this is to use a wildcard DMARC record for the primary domain, which can apply to all subdomains. For example:

_dmarc.example.com. IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:dmarc@example.com; ruf=mailto:dmarc@example.com; fo=1"
*.example.com. IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:dmarc@example.com; ruf=mailto:dmarc@example.com; fo=1"

By using a wildcard DMARC record, the primary domain's DMARC policy and reporting address can be applied to all subdomains, ensuring alignment and reducing the risk of false negatives.

Another challenge with subdomain delegation is managing DKIM key rotation and deployment. When a subdomain is delegated, it is essential to ensure that the DKIM key used by the subdomain is correctly deployed and rotated. In our experience, we have seen that DKIM key rotation can be a complex process, especially when dealing with multiple subdomains and third-party senders.

To optimise DKIM key management for subdomain delegation, we recommend using a centralised key management system that can handle key rotation and deployment for all subdomains. This approach can help simplify the process and reduce the risk of errors. Also, using a hosted or managed DMARC setup can provide an added layer of protection, as these services often include automated DKIM key rotation and deployment.

In terms of best practices for subdomain delegation, we recommend the following:

• Use a wildcard DMARC record for the primary domain to ensure alignment with all subdomains.
• Ensure that the subdomain's DMARC record is correctly aligned with the primary domain's record.
• Use a centralised key management system to handle DKIM key rotation and deployment for all subdomains.
• Regularly monitor aggregate reports to identify potential issues with subdomain delegation.
• Consider using a hosted or managed DMARC setup to simplify the process and reduce the risk of errors.

By following these best practices and being mindful of the potential risks and challenges associated with subdomain delegation, organisations can effectively mitigate the risk of false negatives and ensure optimal email deliverability. In our experience, a well-configured subdomain delegation setup can be a powerful tool for maintaining control over email authentication and preventing spam and phishing attacks. However, it requires careful planning, monitoring, and maintenance to ensure its effectiveness.

Practical Strategies for Mitigating False Negatives

Mitigating DMARC false negatives requires a multi-faceted approach, centreing on the effective management of third-party senders and subdomain delegation. One key strategy is to optimise SPF records to account for all legitimate senders, including third-party services. This involves carefully managing the number of included domains and IP addresses to avoid exceeding the 10 lookup limit. For instance, consider a company that uses multiple third-party senders, such as Mailchimp and Salesforce, in addition to their own mail servers. Their SPF record might look like this:

v=spf1 include:_spf.mailchimp.com include:_spf.salesforce.com ip4:192.0.2.1 ip4:198.51.100.1 -all

In a hosted setup, such as the one we manage at DMARC Engine, we often see customers struggling to keep their SPF records up to date, particularly when dealing with a large number of third-party senders. To address this, we recommend implementing a automated process for updating SPF records, such as using a script to periodically fetch the latest IP addresses from each third-party sender.

Another crucial aspect is DKIM key management. When delegating subdomains, it is essential to use unique DKIM keys for each subdomain to prevent key compromise. For example, if a company delegates the subdomain marketing.example.com to a third-party sender, they should use a separate DKIM key for that subdomain, like this:

default._domainkey.marketing.example.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCkC1W3Z1Q7l5x5R6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K5U6K"

In our experience, managing DKIM keys for multiple subdomains can become complex, particularly when dealing with a large number of third-party senders. To simplify this process, we recommend using a centralised key management system, such as the one we provide at DMARC Engine, which allows customers to easily manage and rotate their DKIM keys.

Subdomain delegation itself also requires careful consideration. When delegating a subdomain to a third-party sender, it is vital to ensure that the sender is configured to use the correct DKIM key and SPF record. For instance, if a company delegates the subdomain news.example.com to a third-party sender, they should ensure that the sender is using the correct DKIM key and SPF record, like this:

news.example.com. IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:aggrep@example.com; ruf=mailto:forensicp@example.com; fo=1"

In a managed setup, such as the one we provide, we often see customers struggling to configure their subdomain delegation correctly. To address this, we recommend using a templating system to simplify the configuration process, such as using a standard template for each subdomain delegation.

Another effective strategy for mitigating false negatives is to implement a robust monitoring and reporting system. This involves regularly reviewing aggregate reports to identify potential issues and taking prompt action to address them. For example, if a company notices a high rate of false negatives from a particular third-party sender, they can take steps to investigate and resolve the issue, such as updating their SPF record or rotating their DKIM key. In our experience, regular monitoring and reporting are critical to maintaining a high level of DMARC compliance, particularly when dealing with a large number of third-party senders.

In addition to these strategies, it is also essential to consider the colour of the DMARC policy, which can have a significant impact on the effectiveness of the policy. For instance, a policy with a low percentage of messages subject to DMARC (pct) may not be effective in preventing spoofing, while a policy with a high percentage may result in a high number of false positives. In our experience, the optimal percentage of messages subject to DMARC will vary depending on the specific use case and the level of risk tolerance. As such, we recommend regularly reviewing and adjusting the DMARC policy to ensure it is optimised for the specific use case.

Finally, it is crucial to consider the operational implications of implementing DMARC, particularly when dealing with a large number of third-party senders. This involves ensuring that all stakeholders are aware of the DMARC policy and its implications, as well as establishing clear procedures for managing and updating the policy. In our experience, effective communication and change management are critical to ensuring a smooth implementation of DMARC, particularly in complex environments with multiple stakeholders. By following these practical strategies, organisations can effectively mitigate DMARC false negatives and maintain a high level of email security and compliance.

Configuring SPF for Third-Party Senders: A Step-by-Step Guide

Configuring SPF for third-party senders is a critical step in mitigating DMARC false negatives, as it enables organisations to authorise legitimate senders while preventing unauthorised ones from sending emails on their behalf. In a hosted or managed setup, such as the one we operate at DMARC Engine, we often see customers struggling to optimise their SPF configurations for third-party senders. To illustrate the process, let us consider a real-world example.

Suppose we have a customer, example.com, that uses a third-party email service provider, mailchimp.com, to send marketing emails. To configure SPF for this third-party sender, example.com needs to add mailchimp.com's IP addresses to their SPF record. The first step is to identify the IP addresses used by mailchimp.com. This information can usually be found in the email service provider's documentation or by contacting their support team.

For instance, mailchimp.com may provide the following IP addresses:

include:_spf.mailchimp.com

This include mechanism allows example.com to authorise mailchimp.com's IP addresses in their SPF record. The next step is to add this include mechanism to example.com's existing SPF record. Let us assume example.com's current SPF record looks like this:

v=spf1 a mx ip4:192.0.2.1 ip4:198.51.100.1 -all

To add the mailchimp.com include mechanism, example.com would update their SPF record to:

v=spf1 a mx ip4:192.0.2.1 ip4:198.51.100.1 include:_spf.mailchimp.com -all

It is essential to note that the include mechanism can be used to authorise multiple third-party senders. However, this can lead to a longer SPF record, which may exceed the 255-character limit imposed by DNS. To avoid this issue, organisations can use the ip4 and ip6 mechanisms to specify individual IP addresses or CIDR blocks.

For example, if mailchimp.com provides a list of IP addresses in CIDR notation, such as 205.201.128.0/20, example.com can add these IP addresses to their SPF record using the ip4 mechanism:

v=spf1 a mx ip4:192.0.2.1 ip4:198.51.100.1 ip4:205.201.128.0/20 -all

When configuring SPF for third-party senders, it is crucial to consider the trade-offs between security and complexity. A more comprehensive SPF record can provide better protection against unauthorised senders, but it may also increase the risk of false positives. To strike a balance between these competing factors, organisations should regularly review and update their SPF records to ensure they remain accurate and effective.

In a hosted or managed setup, such as DMARC Engine, we provide tools and expertise to help customers optimise their SPF configurations for third-party senders. Our platform allows customers to easily add or remove third-party senders, as well as monitor their SPF records for errors or inconsistencies. We also provide guidance on best practices for configuring SPF, such as using the include mechanism and specifying individual IP addresses or CIDR blocks.

To further illustrate the process, let us consider another example. Suppose example.com uses a third-party customer support platform, zendesk.com, to send support emails. To configure SPF for this third-party sender, example.com needs to add zendesk.com's IP addresses to their SPF record. After identifying the IP addresses used by zendesk.com, example.com can update their SPF record to include the necessary include mechanism or IP addresses.

For instance, if zendesk.com provides the following IP addresses:

include:_spf.zendesk.com

example.com can add this include mechanism to their SPF record:

v=spf1 a mx ip4:192.0.2.1 ip4:198.51.100.1 include:_spf.mailchimp.com include:_spf.zendesk.com -all

By following these steps and considering the trade-offs between security and complexity, organisations can effectively configure SPF for third-party senders and mitigate DMARC false negatives. In the next section, we will discuss DKIM key management for subdomain delegation, which is another critical aspect of mitigating false negatives.

In our experience operating a hosted DMARC platform, we have seen that many organisations struggle to manage their DKIM keys, particularly when delegating subdomains to third-party senders. To address this challenge, we provide automated DKIM key management tools and expertise to help customers optimise their DKIM configurations. By combining effective SPF and DKIM configurations, organisations can significantly reduce the risk of DMARC false negatives and improve the overall security of their email ecosystem.

To centre our discussion on the practical aspects of configuring SPF for third-party senders, let us summarise the key takeaways. Firstly, organisations should identify the IP addresses used by their third-party senders and add these IP addresses to their SPF record using the include mechanism or by specifying individual IP addresses or CIDR blocks. Secondly, organisations should regularly review and update their SPF records to ensure they remain accurate and effective.

Finally, organisations should consider the trade-offs between security and complexity when configuring SPF for third-party senders. By striking a balance between these competing factors, organisations can mitigate DMARC false negatives and improve the overall security of their email ecosystem. In our hosted DMARC platform, we provide tools and expertise to help customers navigate these complexities and optimise their SPF configurations for third-party senders.

In terms of colour coding and visualisation, our platform provides a colour-coded dashboard to help customers quickly identify potential issues with their SPF configurations. For instance, if a customer's SPF record is missing a critical include mechanism, our platform will highlight this issue in red, indicating a high-priority error. Conversely, if a customer's SPF record is correctly configured, our platform will display a green tick, indicating a low-risk configuration.

By using this colour-coded system, customers can quickly optimise their SPF configurations and mitigate DMARC false negatives. Also, our platform provides automated alerts and notifications to inform customers of potential issues with their SPF configurations, allowing them to take prompt action to address these issues.

In conclusion to this section, configuring SPF for third-party senders is a critical step in mitigating DMARC false negatives. By following the steps outlined in this section and considering the trade-offs between security and complexity, organisations can effectively configure SPF for third-party senders and improve the overall security of their email ecosystem. In the next section, we will discuss DKIM key management for subdomain delegation, which is another critical aspect of mitigating false negatives.

However, we will organise the discussion around the practical aspects of DKIM key management, rather than reiterating the basics of DKIM. Our goal is to provide actionable guidance and real-world examples to help organisations optimise their DKIM configurations and mitigate DMARC false negatives.

To achieve this goal, we will provide concrete examples of DKIM key management, including real record snippets and code blocks. We will also discuss the trade-offs between security and complexity, as well as the

DKIM Key Management for Subdomain Delegation: Best Practices

When implementing subdomain delegation for DMARC, DKIM key management becomes a critical aspect to consider, as it can significantly impact the effectiveness of your DMARC setup. A well-planned DKIM key management strategy is essential to optimise the security and deliverability of your emails. In a hosted or managed setup, such as the one we operate at DMARC Engine, we centre our approach around flexibility, scalability, and ease of management.

To begin with, it is crucial to understand the importance of using unique DKIM keys for each subdomain. This approach helps to prevent unauthorised use of your domain and reduces the risk of key compromise. For instance, if you have a subdomain sub.example.com and you want to delegate it to a third-party sender, you should generate a new DKIM key pair specifically for that subdomain. This can be achieved by creating a new DKIM key record, such as:

default._domainkey.sub.example.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4hxfBAV3NvDf8aN8to5BV4i6xX4M0a6bD1H0y5fK4nT4Q9jJQK6mZ+rGv9B0pT9qQZfHcLZ4F6xKuT9wQIBCAgCfQw0gDhghCz0N0N0N0N0N0NjIwMDA1MDA1MDA1MDA1MDA1MDA1MDAz"

In this example, default._domainkey.sub.example.com is the selector, v=DKIM1 specifies the version, k=rsa indicates the key type, and p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4hxfBAV3NvDf8aN8to5BV4i6xX4M0a6bD1H0y5fK4nT4Q9jJQK6mZ+rGv9B0pT9qQZfHcLZ4F6xKuT9wQIBCAgCfQw0gDhghCz0N0N0N0N0N0NjIwMDA1MDA1MDA1MDA1MDA1MDA1MDAz is the public key.

When managing DKIM keys for subdomain delegation, it is vital to consider the key size and rotation. A larger key size, such as 2048 bits or 4096 bits, provides better security, but it also increases the computational overhead. In our experience, a 2048-bit key size strikes a good balance between security and performance. Regarding key rotation, it is recommended to rotate DKIM keys every 6-12 months to minimise the impact of a potential key compromise. However, this can be a complex process, especially when dealing with multiple subdomains and third-party senders.

To simplify DKIM key management, we recommend implementing an automated key rotation process. This can be achieved through scripting or using a managed service, such as DMARC Engine, which handles key rotation and management on behalf of our customers. For example, we use a combination of automated scripts and manual oversight to ensure seamless key rotation and minimal disruption to email deliverability.

Another important aspect to consider is the use of a DKIM key management protocol, such as DNS-based Authentication of Named Entities (DANE). DANE allows you to specify the DKIM key record in the DNS, which helps to prevent man-in-the-middle attacks and ensures that the correct DKIM key is used for authentication. To implement DANE, you need to create a TLSA record, such as:

_25._tcp.default._domainkey.sub.example.com. IN TLSA 0 0 1 3082010a0282010100b5a5f8f16c6d696e6b656c6c6f776f726c64

In this example, _25._tcp.default._domainkey.sub.example.com is the TLSA record, 0 0 1 specifies the usage, selector, and matching type, and 3082010a0282010100b5a5f8f16c6d696e6b656c6c6f776f726c64 is the certificate association data.

In addition to DANE, it is essential to monitor DKIM key usage and adjust your strategy accordingly. This can be achieved by analysing aggregate reports and identifying potential issues, such as DKIM key misconfiguration or unauthorised use. For instance, if you notice a high rate of DKIM failures for a specific subdomain, you may need to investigate and adjust the DKIM key configuration or rotate the key to prevent further issues.

In a hosted or managed setup, such as DMARC Engine, we provide our customers with detailed aggregate reports and expert analysis to help them identify and address potential DKIM key management issues. Our team of experts works closely with customers to optimise their DKIM key management strategy and ensure the best possible email deliverability.

In conclusion to this section, effective DKIM key management is critical for subdomain delegation and DMARC implementation. By using unique DKIM keys for each subdomain, implementing automated key rotation, and utilising DKIM key management protocols, such as DANE, you can optimise the security and deliverability of your emails. As a senior email-deliverability engineer at DMARC Engine, I strongly recommend prioritising DKIM key management and seeking expert guidance to ensure the best possible outcome for your organisation.

Interpreting Aggregate Reports: Identifying False Negatives

Interpreting aggregate reports is a crucial step in identifying false negatives, which can be a significant challenge in DMARC implementation. At DMARC Engine, we have seen numerous cases where false negatives have led to legitimate emails being blocked or flagged as spam. To mitigate this issue, it is essential to understand how to read and analyse aggregate reports effectively.

One of the primary challenges in interpreting aggregate reports is the sheer volume of data they contain. A typical aggregate report can include hundreds or even thousands of rows of data, making it difficult to identify trends and patterns. To overcome this challenge, we use a combination of automated tools and manual analysis to identify potential issues. For example, we use scripts to parse the reports and extract relevant information, such as the number of emails that failed DMARC authentication, the reasons for the failure, and the IP addresses involved.

When analysing aggregate reports, it is essential to look for signs of false negatives, such as emails that have failed DMARC authentication despite being legitimate. One common cause of false negatives is misconfigured SPF records. For instance, if an organisation has a third-party sender that is not included in their SPF record, emails sent by that sender may fail DMARC authentication.

v=spf1 include:_spf.example.com -all

In this example, the SPF record only includes the _spf.example.com subdomain, which may not cover all the IP addresses used by the organisation's third-party senders. To mitigate this issue, it is essential to ensure that all third-party senders are included in the SPF record.

Another common cause of false negatives is DKIM key management issues. For example, if an organisation uses a third-party email service provider that uses a different DKIM key, emails sent by that provider may fail DMARC authentication.

k1._domainkey.example.com. 300 IN TXT "k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4mV+Km8m4mGojwHsP0oPfJ1kJHx9t8J0q7X7T5iQj5Km8m4mGojwHsP0oPfJ1kJHx9t8J0q7X7T5iQj5K"

In this example, the DKIM key is not properly configured, which can cause emails to fail DMARC authentication. To mitigate this issue, it is essential to ensure that all DKIM keys are properly configured and aligned with the organisation's DMARC policy.

Subdomain delegation is another area where false negatives can occur. When an organisation delegates a subdomain to a third-party sender, it is essential to ensure that the subdomain is properly configured to use the correct DMARC policy. For example, if an organisation delegates the subdomain.example.com subdomain to a third-party sender, the subdomain's DMARC record should be configured to use the same policy as the parent domain.

_subdomain.example.com. 300 IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:aggrep@example.com; ruf=mailto:forep@example.com; fo=1"

In this example, the subdomain's DMARC record is not properly configured, which can cause emails to fail DMARC authentication. To mitigate this issue, it is essential to ensure that all subdomains are properly configured to use the correct DMARC policy.

In a hosted or managed setup, such as DMARC Engine, we handle the configuration and analysis of aggregate reports on behalf of our customers. This includes identifying potential issues, such as false negatives, and providing recommendations for mitigation. We also provide automated tools and scripts to help customers parse and analyse their aggregate reports. For example, we use a custom-built script to extract relevant information from the reports and provide a detailed analysis of the results.

When interpreting aggregate reports, it is also essential to consider the impact of DMARC policy on email deliverability. A strict DMARC policy, such as p=reject, can cause legitimate emails to be blocked if they fail authentication. On the other hand, a more relaxed policy, such as p=none, may allow spam emails to be delivered. To mitigate this issue, it is essential to monitor the aggregate reports closely and adjust the DMARC policy as needed.

In addition to monitoring aggregate reports, it is also essential to monitor email deliverability metrics, such as bounce rates and complaint rates. These metrics can provide valuable insights into the effectiveness of the DMARC policy and help identify potential issues. For example, if an organisation notices a high bounce rate, it may indicate that the DMARC policy is too strict and is causing legitimate emails to be blocked.

To optimise the DMARC policy and mitigate false negatives, it is essential to use a combination of automated tools and manual analysis. Automated tools can help parse and analyse the aggregate reports, while manual analysis can provide a more detailed understanding of the results. For example, we use a combination of automated tools and manual analysis to identify potential issues and provide recommendations for mitigation.

In terms of concrete recommendations, we suggest the following:

• Monitor aggregate reports closely to identify potential issues, such as false negatives.
• Use automated tools to parse and analyse the reports.
• Adjust the DMARC policy as needed to mitigate false negatives.
• Monitor email deliverability metrics, such as bounce rates and complaint rates.
• Use a combination of automated tools and manual analysis to optimise the DMARC policy.

By following these recommendations and using a combination of automated tools and manual analysis, organisations can effectively mitigate false negatives and improve email deliverability. At DMARC Engine, we have seen significant improvements in email deliverability for our customers who have implemented these strategies. For example, one of our customers was able to reduce their bounce rate by 30% by adjusting their DMARC policy and monitoring their aggregate reports closely.

To sum up, interpreting aggregate reports is a crucial step in identifying false negatives and mitigating their impact on email deliverability. By using a combination of automated tools and manual analysis, organisations can effectively identify potential issues and adjust their DMARC policy as needed. At DMARC Engine, we are committed to helping our customers optimise their DMARC policy and improve email deliverability.

Implementing BIMI and MTA-STS to Enhance DMARC Security

To optimise the security posture of our customers' domains, we often recommend implementing BIMI (Brand Indicators for Message Identification) and MTA-STS (Mail Transfer Agent Strict Transport Security). These protocols can significantly enhance the overall security of DMARC (Domain-based Message Authentication, Reporting, and Conformance) by providing an additional layer of authentication and encryption.

In our experience, BIMI is particularly useful for organisations with a strong brand presence, as it allows them to specify a logo that will be displayed in supporting email clients, providing an additional visual cue to recipients that an email is genuine. To implement BIMI, customers need to create a TXT record with a specific format, for example:

default._bimi.example.com. IN TXT "v=BIMI1; l=https://example.com/logo.svg; a=example.com"

This record specifies the version of BIMI being used, the location of the logo, and the domain to which the logo applies. We have found that the key to successful BIMI implementation is ensuring that the logo is correctly formatted and accessible, as email clients will not display the logo if it does not meet specific requirements.

MTA-STS, on the other hand, is a protocol that enables mail servers to communicate their TLS (Transport Layer Security) support and preferences to other mail servers. This helps prevent man-in-the-middle attacks by ensuring that emails are transmitted over an encrypted connection. To implement MTA-STS, customers need to create a number of DNS records, including a TXT record and a policy record, for example:

_mta-sts.example.com. IN TXT "v=STSv1; id=2023022200"

sts.example.com. IN CNAME example.com

.https://example.com/.well-known/mta-sts.txt
v=STSv1
id=2023022200
mode=enforce
mx=mail.example.com
max_age=604800

This policy record specifies the version of MTA-STS being used, the ID of the policy, the mode of operation (either enforce or testing), the mail servers that support TLS, and the maximum age of the policy. We recommend that customers start with testing mode to ensure that MTA-STS is correctly configured before switching to enforce mode.

One of the key trade-offs to consider when implementing MTA-STS is the potential impact on mail delivery. If a mail server does not support TLS, or if the TLS connection cannot be established, emails may be delayed or rejected. To mitigate this risk, we recommend that customers monitor their mail server logs and adjust their MTA-STS configuration as needed.

In a hosted or managed setup, such as the one provided by DMARC Engine, the process of implementing BIMI and MTA-STS is significantly simplified. Our platform provides a user-friendly interface for configuring BIMI and MTA-STS records, and our team of experts is available to assist with any technical issues that may arise. Also, our platform provides real-time monitoring and reporting, enabling customers to quickly identify and respond to any issues that may impact their email deliverability.

To illustrate the benefits of implementing BIMI and MTA-STS, let us consider a real-world example. One of our customers, a large e-commerce company, was experiencing issues with email deliverability due to a high volume of phishing attacks. By implementing BIMI and MTA-STS, the company was able to significantly reduce the number of phishing attacks and improve the overall security posture of its domain. The company's logo was displayed in supporting email clients, providing an additional visual cue to recipients that emails were genuine, and the MTA-STS protocol helped prevent man-in-the-middle attacks by ensuring that emails were transmitted over an encrypted connection.

In terms of operational considerations, we recommend that customers regularly review and update their BIMI and MTA-STS configurations to ensure that they remain aligned with their organisation's security policies and procedures. Also, customers should monitor their mail server logs and adjust their MTA-STS configuration as needed to ensure that mail delivery is not impacted.

In conclusion to this section, implementing BIMI and MTA-STS can significantly enhance the security posture of a domain, providing an additional layer of authentication and encryption. By understanding the trade-offs and considerations involved in implementing these protocols, customers can make informed decisions about how to optimise their email security. As a hosted or managed setup, such as the one provided by DMARC Engine, can significantly simplify the process of implementing BIMI and MTA-STS, we recommend that customers consider this option to ensure that their email security is properly configured and maintained.

Operational Considerations for DMARC at Scale

When managing DMARC for a large number of domains, the centre of attention shifts from basic setup to optimising for scale, reliability, and ease of management. At DMARC Engine, we organise our operations around a few key principles: automate wherever possible, monitor closely, and standardise configurations to minimise the colour of human error.

One of the primary challenges at scale is managing SPF records, which can quickly become unwieldy as the number of third-party senders grows. For instance, consider a domain that uses multiple marketing platforms, each with its own set of IPs. The SPF record for such a domain might look like this:

"v=spf1 include:_spf.google.com include:mailchimp.com include:salesforce.com -all"

This approach works for small to medium-sized setups but can lead to issues at scale, particularly if the included domains have a large number of IPs or if those IPs change frequently. To mitigate this, we recommend using a subdomain delegation strategy for third-party senders, where possible, to keep the main domain's SPF record as simple as possible.

For example, instead of including all third-party sender IPs in the main domain's SPF record, delegate specific subdomains to those senders. This not only simplifies SPF management but also helps in isolating issues when they arise. A delegated subdomain for a marketing campaign might have its own SPF record, such as:

"subdomain.example.com. IN TXT 'v=spf1 ip4:192.0.2.1 ip4:192.0.2.2 -all'"

This approach allows for easier management and troubleshooting, as issues related to the marketing campaign will not affect the main domain's email deliverability.

DKIM key management is another critical aspect of DMARC at scale. With multiple domains and subdomains, keeping track of DKIM keys, their sizes, and rotation schedules can become complex. We advocate for a centralised DKIM key management system that can automate key rotation and ensure that all sending systems are updated with the new keys. A sample DKIM record might look like this:

"selector._domainkey.example.com. IN TXT 'v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCqGKukO1De7zhZj6+H0qtjTkVxwTCpvKe4eCZ0FPqri0cb2JZfXJ/DgYSF6vUpwmJG8wVQZKjeGcjDOL5UlsuusFncCzWBQ7RKNUSesmQRMSGkVb1/3j+skZ6UtW+5u09lHNsj6tQ51s1SPrCBkedbNf0Tp0GbMJDyR4e9T04ZZwIDAQAB';'"

In a hosted or managed setup, such as what we offer at DMARC Engine, DKIM key management is typically handled through a web interface or API, allowing for easy deployment and rotation of keys across all domains and subdomains.

Monitoring and analysis of DMARC aggregate reports (RUA) are also crucial at scale. These reports provide insights into email authentication issues, helping identify false negatives and areas for improvement. However, parsing and interpreting these reports can be time-consuming and require significant expertise. We use automated tools to process RUA reports, looking for trends and anomalies that might indicate issues with DMARC configuration, third-party sender alignment, or other deliverability problems.

For instance, a sudden spike in authentication failures from a specific IP range might indicate a new third-party sender that needs to be added to the SPF record or a DKIM key issue that requires attention. Automated monitoring can quickly pinpoint such issues, allowing for rapid response and minimising the impact on email deliverability.

Implementing BIMI (Brand Indicators for Message Identification) and MTA-STS (Mail Transfer Agent Strict Transport Security) can also enhance DMARC security and deliverability. BIMI allows brands to specify a logo to be displayed next to authenticated emails, improving user trust, while MTA-STS enforces TLS encryption for email transmissions, protecting against eavesdropping and tampering.

At scale, managing these additional protocols requires careful planning and automation. For BIMI, ensuring that logos are correctly formatted and that the BIMI record is properly configured is essential. A sample BIMI record might look like this:

"default._bimi.example.com. IN TXT 'v=BIMI1; l=https://example.com/bimi/logo.svg; a=https://example.com/'"

MTA-STS, on the other hand, involves configuring a policy for mail servers to only accept encrypted connections, which can be done through a TXT record like this:

"mts.sts.example.com. IN TXT 'v=STSv1; id=20160831085700Z;'"

In a managed setup, these configurations can be easily deployed and managed across multiple domains, ensuring consistency and reducing the risk of human error.

In conclusion to this section, managing DMARC at scale requires a structured approach to SPF and DKIM management, automated monitoring of aggregate reports, and careful implementation of additional security protocols like BIMI and MTA-STS. By standardising configurations, automating key management and report analysis, and leveraging hosted or managed services where appropriate, organisations can optimise their DMARC setup for reliability, security, and ease of management.

Related reading

• DMARC Alignment for Domains with Hybrid On-Premise and Cloud Email Infrastructure
• Optimising DMARC for Domains with Frequent IP Address Changes
• DMARC Alignment for Domains with Complex Email Ecosystems