Administration, Security, and Cost should be the most important factors in this decision, and not neccessarily in that order. Security will take a front seat in importance if someone gains access to your email server, at which point they would have full access to your entire network if not set up correctly. Cost is obvious, you dont state the size of the company, but even if its only 25 or so people, you are going to need adequate power to run a server with any sort of long term use as well as fault tolerancy. Administration is important because you need someone internally with the proper training to keep the server up and running and able to maintain it on a regular basis. There is alot more than just building a pc, throwing on a Network OS and installing an email server such as MS Exchange and then just letting it go. You must start out with a well thought out plan and then test and deploy it. If you have any more specific questions you can email me and I would be glad to help you with any information I can.

2003 MCSE, A+ OS/Hardware Certified, Network + Certified

Try the best of both worlds. We get our email from this service
that is very inexpensive. The pluses here are maintained spam and virus control as well as reliabilty. Then at the office we run this
It’s pretty cheap and gives an extra layer of control / support. This way we have the advantages of a local mail server (shared calendar and so on) with the backup of a professional company. Good luck with what you do.