@steve the only stupid questions are the ones that go unasked ...
I was once in a position like you.
There are 5 basic options:
Google Apps - This has a limit of 25GB and your entire domain will have to move to it at a cost of $50/user/year. Scaling in the future should be doable.
Office365 hosted email (Exchange) - Same 25GB limit and same requirement to move your entire domain to it. $48/user/year
Exchange in your physical office - Very expensive and only sensible when you have a solid technical team, a good data center, AND poor Internet connectivity
OS X Mail Server - Kind of crazy ... but could be fun and in theory should work :-)
Linux mail server - Another fun idea but you're really spinning cycles to fix what I would call a non-core business problem.
Email has been solved and you need to focus on a good solid solution that costs you the least amount of time IN THE FUTURE. Hosted solutions will alleviate most security and performance concerns for a low flat fee which is surely less than your cost to the company. This allows you to focus on problems specific to your company and develop your career.
Research the options above, come up with one and back it up with facts and numbers and present it to your boss with authority. If you have confidence, your boss will listen. If he doesn't, do more research until you feel like your decision is unassailable. If he still doesn't listen, seriously start looking for another organization. It's 2013 and you need to seize the day as an ICT person.
-Adam