Twisted Mail contains high-level, efficient protocol implementations for both 
clients and servers of SMTP, POP3, and IMAP4. Additionally, it contains an 
"out of the box" combination SMTP/POP3 virtual-hosting mail server. Also 
included is a read/write Maildir implementation and a basic Mail Exchange 
calculator (depends on Twisted Names).

Twisted Mail is available under the MIT Free Software licence.  

WWW: http://twistedmatrix.com/