the following material is so universal i don't know if its justified referencing it here. but its definitely part of our 'bible' as mail implementers, and i've personally found these documents to be quite readable and useful (not to mention 'normative' for mail and mail servers):
1. RFC 2821 - Simple Mail Transfer Protocol, at http://www.faqs.org/rfcs/rfc2821.html
2. RFC 2822 - Internet Message Format, at http://www.faqs.org/rfcs/rfc2822.html
pretty much the answers to any basic questions we may come up with in the course of implementing the core of the jboss mail server (i.e., mail messages and SMTP) can be found in these two documents. things like POP, IMAP etc are described elsewhere, but rest on this stuff as a foundation.
Also: any good book on postfix.
Exchange Unleashed (or any good book on Exchange)