Free SMTP Resources
SMTP - A Tutorial
SMTP tutorial covering the Simple Mail Transfer Protocol and including sending a message; SMTP commands; a breakdown of envelope, header and body; and relay agents. Includes some diagrams.
SMTP: Simple Mail Transfer Protocol
"These pages are designed as a reference for the Simple Mail Transfer Protocol (SMTP). They cover today's SMTP implementations and the IETF SMTP specifications, including some SMTP extensions."
SMTP Session Emulator
On-line interactive SMTP session demo/tutorial.
SMTP 101 (SMTP Tutorial)
Short SMTP tutorial at about.com covering the mail transaction, commands and reply codes
All About Email Headers
This is a page at stopspam.org that has a lot of useful information about email headers and some stuff about smtp commands.
Email Protocols : SMTP, MIME, POP and IMAP
These are presentation slides including information about smtp, and the other email protocols.
SMTP RFC
Yahoo listing of RFCs for SMTP.