Fix ‘Mailbox does not exist’ error in Microsoft 365
In order to solve the Microsoft 365 “mailbox does not exist“ error, you must know how a mailbox is created and also how it is attached to a particular Microsoft account through Exchange Online. When a user is created and granted a license in Microsoft 365, a mailbox is created in the background.
But, if the process doesn’t complete, gets interrupted or is slightly out of sync, you may have a user account but the mailbox for that account is not ready to be delivered. This is when you get the “mailbox does not exist” error when using Outlook, Outlook Web App (OWA), PowerShell or using migration tools.
This happens often in user onboarding, directory sync on hybrid, mailbox migration, etc. In the most frequent scenario, the system fails to find an active mailbox for the user account due to problems with provisioning or synchronisation.
Put in its very simplest form: the user exists, but the mailbox wasn’t created, didn’t fully sync, or was not recognised by Exchange Online. Understanding this point will guide troubleshooting efforts to licensing, synchronisation, mailbox GUID validation, and mailbox provisioning instead of client-side issues.
Issues:
- Mailbox does not exist
- Failed to resolve mailbox name
Follow these steps to assign a Microsoft 365 license to the user:
- Login to the Microsoft 365 admin center.
- Go to ‘Users’ and select ‘Active users’ from the left panel.
- Choose the user account you want to license.
- From the 'Licenses and Apps' tab, check all of the license options you would like. Ensure that the selected plan has Exchange Online. For example, choose 'Microsoft 365 E5 Developer'.
- Click 'Save changes'.
- If needed, open the Apps section and enable only ‘Exchange Online’ by selecting its checkbox.
- Click 'Save changes' to apply them.
You can now continue with your migration process. Changes may take time to sync across the Microsoft 365 environment. If the error still appears, wait a bit and reconnect to Microsoft 365 again.
