It seems like every year we do this, we have to rediscover how to setup the DNS entries.  Hopefully this helps...

After talking to students in last year's class they remember configuring MX
records on the DNS server that was in the fishbowl room. Here is an article on
MX records.

Basically you will create a host (A) record that looks like this:

mail.domain.cts 134.39.158.xx xx is the ip address of your exchange box.

Then you will create an MX record that points back to your Host A record.

This is different than what we had been doing which was creating an MX record
with mail in the name. Not sure it makes a huge difference, but two articles
both describe it as follows:

http://demazter.wordpress.com/2010/02/09/exchange-dns-configuration/