This is a very well know issue or you can say not well known for most of the admins
The issue is the auto mapping option
This option
Auto-mapping is an Exchange on-prem & Exchange Online feature, which automatically opens mailboxes with Full Access permissions in a delegate’s Outlook client. The setting is configurable by an Administrator when Full Access permissions are assigned to a user. Once enabled, the periodic Autodiscover requests from the Outlook client will determine which mailboxes should be mapped for a user.
This is the automapping
When you delegate a user permission on a shared mailbox it must appear in the outlook by default you can disable it by PowerShell but it’s enabled by default
The scenario is that a hybrid users (users who have been moved to online exchange in the hybrid environment) will have two problems
The problem
When a user mailbox was on the on-prem exchange and the mailbox had a delegation on the shared mailbox then the mailbox has been moved to online exchange you will face two problems
- The delegation isn’t working anymore cause simply if you try to give delegation to the remote mailbox on the on-prem shared mailbox the user will not be found
-
Another problem is the automapping option will now work and this is related to the Auto Discover option
Solution
Move the shared mailbox online and then make sure that the default reply address is the
sharedmailboxname@yourdomain.onmicrosoft.com
Then wait it will be synced and appeared in your outlook
It will take a while
Or you can just create
new shared mailbox on the exchange with default the choose this domain
sharedmailboxname@yourdomain.onmicrosoft.com
Or you can just log in through your OWA
How to remove auto mapping in shared mailbox
