http://www.faqs.org/rfcs/rfc822.html
RFC822 – STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES
August 13, 1982 – 27 Years old!!!!
http://www.faqs.org/rfcs/rfc2822.html
RFC2822 – Internet Message Format – April 2001
Obsoletes RFC822
Official Standards:
http://tools.ietf.org/html/rfc821
RFC821 – Simple Mail Transfer Protocol
August 1982 – 27 Years old
http://tools.ietf.org/html/rfc2821
RFC2821 – Simple Mail Transfer Protocol – April 2001
Obsoletes RFC821
phpmailer   (http://phpmailer.worxware.com/)
Zend_Mail   (http://framework.zend.com/manual/en/zend.mail.html)
http://www.greenend.org.uk/rjk/2000/05/21/smtp-replies.html
DSN reply codes
http://tools.ietf.org/html/rfc3463
http://en.wikipedia.org/wiki/Bounce_message