Client-Server Mail Protocols FAQ A brief summary of POP3, SMTP, and IMAP, including pointers to sources and frequently-asked-questions sites.
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.
Email Addressing FAQ The user+box@host addressing FAQ. A discussion of what exactly is permissible and verifiable in an e-mail address.
The IMAP Connection University of Washington collection of IMAP documentation and resources.
Internet Mail Consortium Information about IMC and its members, all the Internet mail standards, and links to several IMC-hosted mailing lists.
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.
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.