Determine your public IP address
Create an A record in your public DNS for this IP address.
Use something like mail.domain.com. Wait for this to be done, and resolvable from the outside.
Configure port forwarding through your firewall for ports 25 (SMTP) and 443 (SSL - for OWA).
Verify from the outside that you can telnet to the A record over both ports, and that the server responds. Verify you can get to OWA.
Have the ISP create a reverse DNS record for your public IP address. It should match the A record if possible. Wait for this to be done, and resolvable from the outside.
If you have a 3rd party trusted SSL certificate installed, configure Exchange to use it for TLS.
Change the MX records to point to your A record first, and any backup servers the ISP has second.
Wait at least 48 hours. If all is well, kill your POP3 mailboxes, THEN kill the POP3 connector. IN THAT ORDER, as you want to make sure you get any email sitting in those POP3 mailboxes.
Ideally, you'd use ISA on a server in the perimeter network.
Pat Richard MVP
Plan for performance, and capacity takes care of itself. Plan for capacity, and suffer poor performance.