Question:
Right now we have Exchange set to dial up an ISP and retrieve our mail. We are getting a direct connection to the Internet and I want to host my own Exchange.

What do I need to do to the IMS to support this? Should I click the use DNS check and have my new ISP add DNS MX records to point to my new Exchange server? What about my old ISP; do I need to have them delete the MX record they entered?

Answer:
Yep, you’re on the right track. Have your new ISP create an MX record that points to your new static IP address for your Exchange Server (or proxy if you’re behind one) and set the IMS to use DNS to resolve domain names.

You’ll have to specify the DNS servers that your new ISP recommends, in the TCP/IP Properties of your Exchange Server, of course.

Once you have the live Internet connection and the MX record with your new ISP, you will want to have your old ISP remove the MX record from there so that your mail can find you (I assume you’re keeping the same domain name?) through the new ISP.

In order to prevent a spammer from using your Exchange server for relaying spam, you’ll also want to set the IMS to reroute SMTP e-mail inbound to your domain name, and then click the Routing Restrictions button. Select the second option (to specify the IP addresses of the hosts/machines allowed to relay) but leave the list of machines blank. That will force the IMS to verify that received messages are intended for internal users before accepting them.

Depending upon the specifics of your setup there may be other considerations, but you seem to have the major points covered.