We have moved some of our users' mailboxes from our on-premise Exchange 2003 server to Office 365.
The email addresses listed in the GAL for the users whose mailboxes were moved to Office 365 (on our on-premise Exchange server) have been changed from the actual address:
to one that does not resolve:
The reason for this was apparently that, if the this was not done, senders whose mailboxes still exist on the on-premise Exchange server would have not been able to send emails to the new, Office 365 mailboxes for those users.
In addition, the Office 365 user accounts on the Exchange 2003 server have also been set to "Hide from Exchange address lists", to prevent on-premise Exchange users from accidentally sending emails to these accounts.
The above workaround solves the problem of on-premise Exchange mailbox users getting "message undeliverable" errors when trying to email Office 365 users - but the downside is they cannot find the Office 365 email addresses in the GAL.
Eventually, we hope to migrate all of our users to Office 365 but, in the meantime, I was wondering if there is any way for us to keep users' correct email address in the on-premise GAL without undeliverable errors?