Email system/Bibliography
[Crocker08] D. Crocker (2008) "Internet Mail Architecture", http://tools.ietf.org/html/draft-crocker-email-arch-11. Comprehensive description of the modern Internet mail architecture.
[Klensin08] J. Klensin, ed. (2008) "Simple Mail Transfer Protocol", RFC-5321, http://tools.ietf.org/html/rfc5321. Authoritative reference on SMTP.
[PnD07] L. Peterson, B. Davie (2007) "Computer Networks: A Systems Approach" 4th ed. Sect. 9.1.1 "Electronic Mail", ISBN 0-12-370548-7. A good textbook explanation.
[Resnick08] P. Resnick, ed. (2008) "Internet Message Format", RFC-5322, http://tools.ietf.org/html/rfc5322. Authoritative reference on message format, including headers.
[Stevens94] W.R. Stevens (1994) "TCP/IP Illustrated, vol. 1, The Protocols", Chapter 28, "SMTP", ISBN 0-201-63346-9. Best reference on the underlying TCP/IP protocols.