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Mailbox appears as Mail User in O365
06-Sep-24

I got a report of an NDR when emailing a user in Outlook. I didn't know the history of this account but when I checked in Exchange on-prem it showed as a remote mailbox. However, in O365 the mailbox was showing under 'Contacts' as a Mail User. To fix this run the following:

Exchange On-Prem PowerShell
===========================

Disable-RemoteMailbox SoapJ

wait for AD replication to occur, about 15mins...

Enable-RemoteMailbox SoapJ -RemoteRoutingAddress SoapJ@mydomain.mail.onmicrosoft.com

After another wait for sync to occur you should see the mailbox in O365 again.

Similar to: Mailbox present in O365 but missing on-prem