User can’t access mailbox after migration to Exchange Online

After migrating a user mailbox to Exchange Online (from Exchange 2016) the user could no longer access their mailbox in Outlook, it gave no error just behaved as if it was offline.  I tried removing the profile and creating it again but Outlook just reported that “Something has gone wrong”.  Not very helpful at all!

After a lot of investigation the solution was simple: the RemoteRoutingAddress attribute for the user was wrong.  I was able to check this using the Get-RemoteMailbox PowerShell cmdlet:

Get-RemoteMailbox -identity user@mydomain.com | select *remote*

For some reason, the RemoteRoutingAddress attribute was set to firstname.surname@ourdomain.com rather than firstname.surname@tenant.mail.onmicrosoft.com.  I was able to correct this using the Set-RemoteMailbox cmdlet:

Set-RemoteMailbox -identity user@mydomain.com -RemoteRoutingAddress firstname.surname@tenant.mail.onmicrosoft.com