How to block Win-hosted spamming worms with SuSEfirewall2

Hi, Time to time Windows PCs on our network get infected with spamming worms and Antivirus is not always effective. I think it is simply possible to block those worms with SuSEfirewall2. Here is my idea. All e-mail clients sending e-mails via Linux server which acts as firewall, router, mail and web server. I do not think worms are contacting local smtp server, instead, they use direct smtp connection to distribute junk. If it would be possible to allow internal (client) -> internal (our server) smtp connection while disable internal (client) -> external smtp, worms will be simply blocked. Anyone have an idea if this will work, and how to do it in SuSEfirewall2? Thanks in advance for any suggestion(s). Andrei Verovski