Email system/Bibliography
RFC-5321 (2008), "Simple Mail Transfer Protocol", J. Klensin, ed., http://tools.ietf.org/html/rfc5321. IETF Draft Standard.
RFC-5322 (2008), "Internet Message Format", P. Resnick, ed., http://tools.ietf.org/html/rfc5322. IETF Draft Standard.
RFC-5598 (2009), "Internet Mail Architecture", D. Crocker, http://tools.ietf.org/html/rfc5598. IETF Informational document, a much more detailed look at email system architecture.
[PnD07] L. Peterson, B. Davie (2007) "Computer Networks: A Systems Approach" 4th ed. Sect. 9.1.1 "Electronic Mail", ISBN 0-12-370548-7. Most popular textbook explanation.
L.Peterson, B.Davie (2007). Computer Networks: A Systems Approach, 4th. Morgan Kaufmann. ISBN 0-12-370548-7. Most popular textbook explanation; Sect. 9.1.1 "Electronic Mail"
[Stevens94] W.R. Stevens (1994) "TCP/IP Illustrated, vol. 1, The Protocols", ISBN 0-201-63346-9. Excellent reference on the underlying TCP/IP protocols; chapter 28 on SMTP.
W. Richard Stevens (1994). TCP/IP Illustrated, vol. 1, The Protocols. Addison-Wesley. ISBN 0-20163346-9. Excellent reference on the underlying TCP/IP protocols; chapter 28 on SMTP.