One of the features available during a Hybrid / “rich-coexistence” migration to Office 365 is the friendly URL redirect that users can receive if they try using the old OWA URL that was pointing to the old Exchange server.
However many people may not be aware that using that feature requires establishing a legacy namespace with Exchange 2007, as the Exchange 2013 cannot proxy all workloads for Exchange 2007.
For example, owa.contoso.com would be redirected to the Hybrid server, and then it could proxy the request to a back-end legacy Exchange server.
However, if it is more important to an organization that the impact to mobile phones is reduced, then you can leave the pre-existing namespace (ex: webmail.contoso.com) intact and instead introduce a new external namespace for the Hybrid server (ex: mail.contoso.com) so that it exists side by side with the previous environment.
The benefit is that users only have to change their mobile phone setting once.
The downside with this approach is after a user’s mailbox migration completes, if the user attempts to logon to the old Exchange OWA URL then they will get an error message that their mailbox cannot be found.
This can be mitigated with a communication plan that provides the new OWA URL so that users don’t try to go to the old URL. To me, that’s worth the tradeoff of having to make the user change their mobile device twice: once to legacy.contoso.com and a second time when their mailbox migrates to Office 365.
For example, among the things you can communicate to an end-user prior to their mailbox being migrated:
• You will get a pop-up to restart outlook
• You will be prompted to enter credentials, please use your email address as your username.
• The new URL for Outlook Web Access is http://outlook.com/contoso.com
• Follow these instructions for re-attaching your mobile device to Office 365