Adding an e-mail server

We are currently planning on bringing our webhosting inhouse. We are going to use a PowerEdge 2500 server with 2X1GHz, about 2GB or RAM and 56GB of space. The server right now is being setup to run IIS5 and a survey program. We are considering bringing our e-mail in house as well – probably on a program called SLmail. Will this survey be able to support the web and about 50 users of e-mail or should we setup a second e-mail server – note we are trying to keep cost down some. Thanks for your help.