A client recently implemented Exchange 2010 as a replacement for an existing externally hosted POP3 mail system. The client is relatively small. So, the plan was to have no coexistence and the real domain name was not used for testing. A new domain name was used for testing. Consequently during testing, all of the external URLs were configured as testdomain.com.
During switchover day, we obtained a new certificate with the proper names, including realdomain.com. During testing of OWA, the web browser was redirected to testdomain.com. Aha, I forgot to update the external URL. Exchange 2010 has a nice wizard for updating external URLs for all web services. All fixed up and tested in 10 minutes.
A week later, I get a call from the client indicating that there is a certificate problem and users are being redirected to testdomain.com. After some back and forth communication this is only for Outlook Anywhere clients.
Outlook Anywhere does not have an external URL, but it does have a host name associated with it. This host name is not updated by the wizard that updates all of the external URLs. Under Server Configuration, you need to go into the Properties of the server and modify the host name on the Outlook Anywhere tab. It's a 30 second fix as long as you remember to do it.
See here for detailed instructions: http://technet.microsoft.com/en-us/library/aa996902.aspx