RFC 3848 / START HERE
Mail transport traces, before the details
ESMTP means Extended Simple Mail Transfer Protocol; LMTP means Local Mail Transfer Protocol. This RFC registers names for mail transmission types.
It defines seven labels used after with in a Received mail trace field. They summarize the protocol and successful authentication or STARTTLS properties of one incoming hop.
The idea in one picture
Observe one hop
Record its type
Check the evidence
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Mail operators need a compact way to diagnose how a message travelled between programs. These labels record high-level hop properties, without revealing or proving the details of a security mechanism.
A familiar example
In an email’s source view, with ESMTPSA claims that one ESMTP reception used successful STARTTLS and AUTH. It does not prove who wrote the message, that the header is authentic, or that every later hop was protected.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Find the transport label in a Received fieldESMTP means Extended Simple Mail Transfer Protocol. A receiving server adds a Received trace field; its with clause names the transmission type used on that incoming hop.→
- Record successful authentication with the A suffixSMTP AUTH is the extension used to authenticate a client to a receiving server. ESMTPA records that this authentication succeeded on the ESMTP hop.→
- Combine successful STARTTLS and AUTH without guessingSTARTTLS begins Transport Layer Security (TLS) negotiation on an SMTP connection. The S suffix records it successfully providing a strong transport encryption layer; adding successful AUTH yields ESMTPSA.→
- Use the LMTP family for local deliveryLMTP means Local Mail Transfer Protocol. It is a separate protocol for agreed delivery arrangements, often between a queue manager and a delivery agent. RFC 3848 registers four LMTP transmission types.→
- Use traces for diagnosis, not automatic trustMail can carry several Received fields written by different systems. A readable ESMTPSA token is only a claim about a hop; the header itself does not authenticate the claim.→
This introduction explains RFC 3848 (2004). Read the original RFC ↗