Postfix
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http://www.postfix.org/
http://en.wikipedia.org/wiki/Postfix_(software)
The Postfix official documentation is quite good and comprehensive. After you have done a basic postfix install, you may have a look at the tutorials on how to fully configure your mail server with courier:
http://linux.overshoot.tv/wiki/server/courier
Security
File permissions
/etc/postfix/main.cf should be world-readable!
Basically, any user who is allowed to send mail should be allowed to read this file. This include the users' web applications (group www-data?), and any user script (e.g. a user using PHP's mail() function).
Other files containing DB passwords and other sensitive information should be readably by postfix. That would include all the files referenced by the following configuration items:
main.cf:virtual_mailbox_maps = mysql:/etc/postfix/mysql_mailbox.cf
main.cf:virtual_alias_maps = mysql:/etc/postfix/mysql_alias.cf
main.cf:virtual_mailbox_domains = mysql:/etc/postfix/mysql_domains.cf
main.cf:# transport_maps = mysql:/etc/postfix/mysql_transport.cf