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.

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. Even on a transition I would have used a single IP/SSL certificate. You would have one IP address for Exchange 2010 and then another for the legacy URL on Exchange 2003. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

As Sembee stated, you don't need two external URLs or two IPs on your Exchange 2010 server. You need to setup a legacy name space for the Exchange 2003 server, which can be included in your UC\SAN cert. Then configure Exchange 2010 to redirect any clients on Exchange 2003 to the legacy name space. Then you change mail.xyz.com to point to Exchange 2010. Exchange 2010 will then handle figuring out if the user's mailbox is on 2003 or 2010 and redirect them to legacy.xyz.com if needed. So your UC\SAN cert will have mail.xyz.com, autodiscover.xyz.com, and legacy.xyz.com. You could save a bit of money and deploy a new name space for Exchange 2010, like 2mail.xyz.com, but this would require you update all ActiveSync devices and users to use the new space space. So it's easier to have Exchange 2010 take over the exiting namespace and have it redirect to the new legacy.xyz.com FQDN, which is transparent to ActiveSync and Outlook Anywhere and semi-transparent (URL will be change after the user logins into 2010 OWA) to OWA users. See: Upgrade Exchange 2003 to Exchange 2010 for more details If this post helps to resolve your issue, please click the "Propose as Answer" If you find it helpful , mark it as helpful by clicking on "Vote as Helpful" button at the top of this message. By marking a post as Answered, or Helpful you help others find the answer faster. If you need an expert migration consultant to assist your organization feel free to contact me directly.Microsoft Infrastructure Architect, MCSE: M, MCTIP, Microsoft Exchange MVP izzy.org | jasonsherry.org | Twitter: @JasonSherry