I had this question after viewing Exchange 2010 Coexist with Exchange 2016
I am midway through and Exchange 2010 to Exchange 2016 migration. At this point I have Exchange 2016 set up and configured but not communicating with my external smart host.
If I move a mailbox to Exchange 2016, the mailbox is no longer accessible through the standard owa address, email.mydomain.com. If a user connects to their mailbox after it is moved to Exchange 2016, they are prompted for their username and password each time. Additionally, many of our users have full mailbox access to other users. They are no longer able to access mailboxes added to their Outlook client that still exist on 2010.
I plan to completely transition to Exchange 2016 and remove my Exchange 2010 server but I'm looking to move as many mailboxes as possible before trying to complete the transition as I know that the actual moving of the mailboxes will take some time.
I am able to send and receive e-mail from a mailbox on Exchange 2016 so it appears that the mail flow is set up correctly. How can I allow access to both mailbox servers during my transition? Also, we have many iPhone/Android users using activesync. Eventually the 2016 server will have the same cert as well as the same external DNS same as the old 2010 server. Will it be necessary to reconfigure the mobile users after moving their mailboxes to 2016?
Also, internal and external url's on the Exchange 2010 server point to (as an example, mail.mydomain.com). On the exchange 2016 server we have all internal url's pointed to (exchange2016.mydomain.loc
al) while external url's point to mail.mydomain.com.