• https://darkmail.info/

• http://cr.yp.to/mail.html

• Message Transfer Agent(MTA)

• http://james.apache.org/server/
• https://code.google.com/p/subethasmtp/

• http://javaemailserver.sourceforge.net/
• http://quintanasoft.com/dumbster/
• http://jpxx.sourceforge.net/

Related: Study_Security_Mail