These days it's easier and cheaper to outsource email services. I've worked with email servers in the past, and they are a real pain to deal with, especially if you are the only one responsible for troubleshooting issues and keeping it running. I fully agree with what @wwb_99 said above.
Heck, you can get fully-blown hosted exchange accounts with IMAP, POP, webmail, mobile webmail, activesync, 25GB of storage(per account) and full support for around $7-$10/mo per account. Plus, not having to deal with a lot of the technical headaches of running your own server is a big bonus.
I've actually gone through three major migrations with three different organizations and migrated from an in-house email server to an external 3rd-party email host. Each time, I went through the list of pros and cons and cost analysis, and each time hosted email services ended up being a no-brainier. While the migrations were headaches in and of themselves, after they were completed, I rarely had to do much other than create and delete accounts.
Running your own email server will suck up a lot of time, so unless you're just doing it as a learning exercise, I seriously do not recommend it.