An Email Service Provider (ESP) is a third-party platform that sends email for you, covering marketing campaigns, newsletters, transactional notifications and other automated mail. Common examples are services that handle bulk sending, list management and delivery on behalf of your domain.
ESPs matter for email authentication because each one sends as your domain from its own infrastructure. For that mail to pass DMARC, you must authorise the provider correctly: add their sending hosts to your SPF record (often via an include:) and publish their DKIM public key under the selector they assign. DMARC then requires that SPF or DKIM not only passes but is aligned with the visible From domain.
The most common pitfall is exceeding the SPF 10-lookup limit when several ESP includes stack up, which causes a permerror and breaks SPF entirely. Many providers also use a custom Return-Path subdomain, so DKIM alignment is usually the more reliable signal.
To check your ESPs, list every service that sends as your domain, confirm each is present in SPF and has a working DKIM selector, then review aggregate reports for sources that fail alignment. DMARC Engine's done-for-you enforcement maps out each ESP and keeps these records aligned for you.
Check it on your domain
- SPF Checker: validate SPF and count DNS lookups against the limit of 10.
- DKIM Checker: look up and validate a DKIM public key by selector.
- DMARC Report Analyser: read a DMARC aggregate (RUA) report in plain English.