SMTP (Simple Mail Transfer Protocol) is the internet standard for transferring email between mail servers and from mail clients to servers. It defines the commands and replies, such as HELO/EHLO, MAIL FROM, RCPT TO and DATA, that move a message from sender to recipient.
SMTP matters because nearly all email transport depends on it. A sending server finds the recipient domain's MX record, opens an SMTP session, and hands over the message. Two address concepts are important: the SMTP MAIL FROM, also called the envelope sender or Return-Path, which SPF checks, and the visible From header that recipients see. These can differ, which is part of why DMARC alignment exists.
SMTP itself offers no built-in encryption or sender verification. STARTTLS upgrades a session to an encrypted channel, while authentication standards layered on top prove a message is genuine.
To check SMTP, you can connect to a mail host on port 25 (or submission ports 587 and 465) and confirm it responds, advertises STARTTLS, and accepts mail for your domain. Verifying that your sending services produce aligned SPF and DKIM results is the key step for trustworthy delivery.
Check it on your domain
- SMTP Test: resolves a domain's mail servers and probes SMTP.
- MX Record Checker: looks up the mail servers for a domain.
- DNS Record Checker: looks up any DNS record type for a host.