I currently work for a small company using Exchange 2003 for their email server. All email is pulled in from a POP3 multidrop account with no filtering other than Exchanges own IMF.
I am looking to create a linux based mail proxy that will accept the pulled in POP3 messages via SMTP, scan via spamassassin and then pass those marked up mails onto the Exchange servers smtp port for delivery as normal. I'll then filter them based on the X-Spam-Status header.
There in only one email domain in use.
Can someone please guide me through the setup of such a system? I'd prefer to use Ubuntu linux since I am most familiar with that, although I am not familiar with the setup of any linux MTA's.