Getting a mailbox not found error in Outlook on the web?

If you're using Outlook on the web and you get a Mailbox couldn't be found for error, the account that you used to connect to Outlook on the web doesn't have an Exchange Online license and therefore, no mailbox is associated with the account.

Assign a license to your account

Your admin can assign a license to your account by following these steps:

1. Go to the Microsoft 365 admin center. In the left nav pane, in the Users section, select Active users, and then select the user who is seeing the error.
2. In the user page that opens, go to the Licenses and Apps section, select the appropriate Location value, and assign a license that contains Exchange Online (expand Apps to see its details).
3. When you're finished, click Save changes.

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