IMail Server

I've just installed IMail Server 8 on an old WIn2000 Workstation which is running on a Win2000Server Domain. The Win2000 workstation connects through a router to the outside world. The IMail Server seems to be setup correctly, all users can send out mail but cannot receive from the outside. They can send mail internally. This system is very new to me, as far as I'm aware it had Exchange Server running before. Also the email addresses have changed but I don't see that as a problem. Could it be the ISP which handles the domain name? This system has about 4 weeks worth of emails sitting somewhere and I don't see why they aren't flooding down. Even if the email addresses have changed should they not go to the postmaster? This is very new to me, any help would be appreciated.