message transfer agent
software that transfers electronic mail messages from one computer to another using SMTP
Dovecot
free software IMAP and POP3 server
Microsoft Exchange Server
calendaring and mail server developed by Microsoft that runs exclusively on the Microsoft Windows Server product line
Bongo
free software
Mercury Mail Transport System
email server
Apache James
open source mail server
Exim
mail transfer agent
Postfix
mail transfer agent
Courier Mail Server
mail-server software suite