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