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Sounds like the router has an internal translation where it takes anything sent to mail.domain.com and/or e-mail related ports and routes it to a local IP. It's hard to give advice on that without knowing more specifics though.
As for the mail server, I'd suggest running sendmail under Linux. If it's not the most widely used mail server, it's certainly up there. qmail is gaining ground, but I think it's a tremendous pain in the ass to use in comparison for small networks.
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