I have a single CAS server handling OWA and Activesync for our new Exchange 2010 implemenation. We're migrating from an Exchange 2003 system which has frontend server installed. The 2003 frontend server's external name is mail.XYZ.com, while the Exchange 2010's is 2mail.XYZ.com. Is it possible to add a second External URL to the Exchange 2010 server, so it can handle both mail.XYZ.com and 2mail.XYZ.com Activesync traffic? It it can, it would help us migrate our smartphone users over to the new Exchange 2010 environment.

No. The external URL entry is really only used by autodiscover. Therefore as long as you have the old name as one of the additional names in the SSL certificate, then it will continue to work correctly. Depending on whether you are moving everything across in a "big bang" migration or staged you could continue to use the old name. The name of the server can have no connection to the URLs used by Exchange. Simon. Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

Hmm, I should have added some more about the configuration of our CAS. It has 2 IP addresses assigned, each one routing to a unique external IP. One IP services mail.XYZ.com and the other services 2mail.XYZ.com. There are two SSL certs installed and bound in IIS to the appropriate IP. This way 2mail.XYZ.com is usable now, and we simply need to reconfigure our firewall to start routing external traffic to the 2010 mail.XYZ.com instead of the 2003 mail.XYZ.com. So, if I understand correctly, I should be able to use both "mail" and "2mail" on the 2010 CAS at the same time without issue. Correct?

Why complicate it so much? Single Unified Communications certificate with a single IP address. All required names on the same SSL certificate. You can use both names at the same time, because they resolve to the correct place. However I would be looking to put it back to a single IP/SSL certificate. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

We plan to put it back to a single IP/SSL certificate as soon as possible. It's just a temporary solution to easy our transition.