Many people have documented converting a user mailbox into a shared one with the quick link in the Exchange Admin section of Office 365 but converted mailboxes remain in the user list with the status of “Synced with Active Directory”

Because of this is that even though you free up an Office 365 license if you delete the user from your on premise server you lose the shared mailbox. Having a bunch of inactive users in your local AD just to keep shared mailboxes is annoying and potentially confusing unless you create a new OU for them. The most common suggestion I’ve seen for being able to delete them is to export the mailbox to a PST using Outlook and then copying the contents back to a newly created shared mailbox with the same name afterwards.

During the process of doing some clean up work it occurred to me to try restoring one of these mailboxes from the “Deleted Users” screen. I already had the local PST so if it didn’t work it would be OK. The result was a normal user mailbox with the Status “In Cloud” and assigned a license.  Once again I converted this mailbox back to a shared one and removed the license to end up with a Shared mailbox that isn’t associated with an AD user.

Here were the steps I followed:

1. Export to PST (doing this on the user’s computer by taking control of the account is the easiest)
2. Convert User Mailbox to Shared (status is still Synced)
3. Delete user from AD
4. Force AD Sync
5. Restore user from Deleted Users (recreated user status is In Cloud)
6. Convert User Mailbox to Shared
7. Remove License.

Step 2 may not be necessary and I’ll update this when I have an opportunity to test.

This may be a lucky hack so don’t try this without having your backup first!