If your PC computers are using Windows, they should either have an add in, or built-in TCP stack. Your PCs will also need to have a POP3 Mail client like Eudora installed. If you are using Windows 95, you need to install the TCP stack on each of the clients. Usually the default installation does not install TCP, so you need to install it manually.
Since you want an internal e-mail system, you can use IP addresses to identify all of the computers. If you use IP addressed, you should disable DNS (domain name server) when you setup the TCP properties on the PCs. If you want to use domain names, you will need to run a DNS on one of the computers.
MacDNS, a DNS application, can be run on the same Macintosh computer as AIMS. With a DNS users can refer to others as "user@mycompany.com" as oppose to "user@192.0.0.0"
If you have trouble setting up the TCP/IP connection in Windows, please contact Microsoft for further support.