Hey Matt,
Exchange is a very powerful and highly configurable server package, but have never heard of Rackspace.
We use Domino (Lotus Notes) which is also very powerful and probably even more configurable. It even has it's own designer and language, but requires more maintenance. It also costs a heck of a lot of money and you are charged per user basis. Some Outlook users find it hard to adapt to a Lotus Notes client.
Another one to consider is MailEnable. If your just using POP and SMTP, then you can download the Standard version which is completely free, altough much less configurable than the above, as it's a much smaller package. It runs on any version of Windows including non server packages such as XP, and requires virtually no maintenance. It's well worth a look.
MailEnable Downloads (you can also purchase Standard and Enterprise versions of the software if you require more for your company).