http://www.ietf.org/rfc/rfc2821.txt
This document is a self-contained specification of the basic protocol
for the Internet electronic mail transport. It consolidates, updates
and clarifies, but doesn't add new or change existing functionality
of the following:
- the original SMTP (Simple Mail Transfer Protocol) specification of
RFC 821 [30]
- domain name system requirements and implications for mail
transport from RFC 1035 [22] and RFC 974 [27]
-
the clarifications and applicability statements in RFC 1123 [2]
-
material drawn from the SMTP Extension mechanisms [19]