Internet Mail Consortium

Information about IMC and its members, all the Internet mail standards, and links to several IMC-hosted mailing lists.
http://www.imc.org/

Message Access Paradigms and Protocols

Provides extensive background and compares the Post Office Protocol (POP) with the Internet Message Access Protocol (IMAP).
ftp://ftp.cac.washington.edu/mail/imap.vs.pop

Email Addressing FAQ

The user+box@host addressing FAQ. A discussion of what exactly is permissible and verifiable in an e-mail address.
http://faqs.org/faqs/mail/addressing/

The IMAP Connection

University of Washington collection of IMAP documentation and resources.
http://www.imap.org

MIME Meta-FAQ

comp.mail.mime meta-FAQ: Help for MIME problems. Pointers to information about Multipurpose Internet Mail Extensions and some answers to common problems with MIME.
http://www.faqs.org/faqs/mail/mime-faq/mime0/

Detailed Revision/Update of Message Standards

The charter of the DRUMS IETF working group. DRUMS works on clarifying, codifying, and extending existing Internet messaging standards.
http://www.ietf.org/html.charters/drums-charter.html

Network Services and Applications

A series of lecture notes and slides from the Faculty of Informatics and Communication, Central Queensland University. Covers many mail protocols.
http://www.infocom.cqu.edu.au/Units/aut2000/85321/Resources/Lectures/2000/16/

Client-Server Mail Protocols FAQ

A brief summary of POP3, SMTP, and IMAP, including pointers to sources and frequently-asked-questions sites.
http://www.faqs.org/faqs/mail/mailclient-faq/