The definitive source for postifx and its documentation: http://www.postfix.org/
Archive site for the postfix mailing list: http://msgs.securepoint.com/postfix/
E-mail architecture in a nutshell, plus excellent links to e-mail software and information: http://www.members.aa.net/~swear/pedia/mail.html
RFC #821, “Simple Mail Transfer Protocol”. Useful for making sense of mail logs, SMTP headers, etc.: http://www.ietf.org/rfc/rfc0821.txt