Bringing email in-house, What to use?

Our company is thinking of bringing email in-house. We?ve setup a test environment and used Mail-Enable and it worked. We used 10 user accounts to test to make sure we had our MX records, A records, and DNS settings were configured correctly. I?ve looked around for opinions, and of course there?s Exchange. Kerio seemed to have a big following. I need something that doesn?t need a whole lot of administration since there is only two system administrators, and we have to travel quite a bit. We?ll start with 250 email users but will grow to about 400 email users. I?m looking for any experience that you may have with your environment that you either administer or use. I searched the forum, but I couldn?t quite find what I was looking for. If this topic has been discussed (most likely it has), I apologize for not finding it. B-)