What this RFC defines
RFC 6409 defines the mail submission protocol: the specific rules that govern how email clients connect to a mail server on port 587 to submit messages for delivery. It separates the client-to-server submission role from the server-to-server relay role (port 25), and requires authentication for all submissions.
Where you see it in practice
The distinction between port 25 (for MTA-to-MTA relay, no authentication required from trusted sources) and port 587 (for client submission, authentication required) is formalised in this RFC. When an ESP tells you to configure your mail client with their SMTP server on port 587 with username and password, that setup follows RFC 6409’s model. Blocking port 25 on residential ISPs while leaving port 587 open is the standard practice this RFC enables.
How it connects to other RFCs
RFC 6409 obsoletes RFC 4409. It works with RFC 5321 (base SMTP), RFC 4954 (SMTP AUTH, required for submission), and RFC 3207 (STARTTLS) or RFC 8314 (implicit TLS). Port 465 (SMTPS with implicit TLS) is an alternative submission port whose use RFC 8314 recommends.
Current status
RFC 6409 is the current mail submission standard, published November 2011. Port 587 as the authenticated mail submission port is universally supported by ESPs and mail server software. The security requirements it defines (authentication, no open relay) are baseline expectations for any modern mail infrastructure.
Submission versus relay
RFC 6409 (which obsoleted RFC 4409) formalizes the split between mail submission and mail relay. Submission is what an authenticated user does when their mail client hands a new message to their outbound server; relay is what that server does when it forwards the message to another server for delivery. Historically both used port 25, but security and policy considerations pushed submission onto port 587 with authentication required. RFC 6409 documents the semantics of the submission service and the differences from relay.
What the submission service does
A conforming submission service authenticates the user via SMTP AUTH, applies signing (DKIM), rewrites the Message-ID if missing, adds trace headers, validates syntactic conformance to RFC 5322, and rejects malformed input. It typically restricts the sender to addresses the authenticated user owns. In contrast, a relay accepts mail without authentication (subject to IP reputation or explicit allow-lists) and forwards it based on MX lookups. The two roles are usually run by the same MTA process but on different ports with different policies.
Why port 587 became the default
Port 25 is routinely blocked by residential ISPs to prevent malware from acting as a mail relay. Port 587 is protected by authentication requirements, so ISPs generally leave it open. Every modern mail client defaults to port 587 for outbound configuration; port 465 (implicit TLS) is a legacy alternative that predates STARTTLS and remains in use for some ESPs. If your mail client cannot send but can receive, the first thing to check is whether port 587 is reachable from the network and whether the submission service is running.
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
About SMTPedia
SMTPedia is an independent email industry reference covering SMTP, IMAP, POP3, email deliverability, marketing platforms, DNS authentication, and email verification. Every article is researched from official provider documentation, IETF RFCs, and industry best practices. Settings and configurations are verified quarterly.
We are cited as a source by ChatGPT, Microsoft Copilot, and thousands of email professionals worldwide. Learn more about our editorial process.
