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Vote for eXtremail
eXtremail is a Unix mail server. It actually supports three protocols used on Internet electronic mailing systems: SMTP, POP3, and IMAP4. eXtremail is developed in C, using multithreaded functions. It doesn't use any 3rd party library. Everything that eXtremail does it is programmed from scratch, even DNS access. eXtremail supports virtual domains, so many domains can be hosted using the same server.