How to create a SMTP server in a Windows 2008 dedicated webserver with a free solution with PHP/Apache?

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http://serverfault.com – I have a Windows 2008 server with these applications installed: Apache: 2.2.21 MySQL: 5.5.27 PHP: 5.3.17 I have to install a Control Panel and a outgoing mail server for the Site that will run in this server. How do I install a SMTP server and what is the free solution compatible with those applications? I quite don't understand the mechanics of smtp servers. (HowTos)