I have a situation where I am migration users from and SBS 2008 (Exchange 2007) to a new Exchange 2013 box. Now I have started the migration of the users and the users that are office based and the are fine as their Outlook when started after the migration picks up the new location of the users mailbox on the new Exchange box and it works fine. The company has a large number of mobile users that connect via OWA and of course use their various mobile devices to read their emails. If I login to the OWA it is the still existing SBS OWA and so I can login as normal with any user that has not been migrated but if I try with a user that has it throws the error ‘Outlook Web Access is currently unavailable.’ My problem is that there are too many mobile users to migrate over a weekend and I cannot afford any downtime for them so I need both migrated users and non-migrated uses to be able to use OWA.
Just as an aside in the Exchange 2013 Admin centre under servers I have not added the External URLs yet for the company or the company’s certificate (not a self-signed one but a proper one)
Hope this makes sense