A spammer doesn't need your box to run smtp to send you spam; they just need access to an smtp host to relay mail to your smtp host. If you're hosting your own mailservers you'll need this port open.
Given the excellent free mailservers available for Windows, *BSD and Linux, it's not unreasonable (as long as it doesn't violate your TOS) to host your own mail. For Windows, »www.mailenable.com
has an excellent package that includes extensive anti-relaying protection.
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