Troubleshooting the ‘Mailbox does not exist’ error

Problem:

You are no longer able to start backup or migration jobs in CodeTwo software or perform other actions that involve connecting with your Microsoft 365 (Office 365) tenant. You receive the following or similar errors:

Unable to perform this command. Mailbox does not exist

Failed to resolve source mailboxes email addresses

Migration process has been terminated. C2ExchangeMigration.Ews.Exceptions.NonExistentMailboxException: Mailbox '<mailbox_name>' doesn't exist.

Solution:

It seems that the admin account you use with CodeTwo software to perform backup or migration tasks is not mailbox-enabled. It is one of the requirements for the admin account used in CodeTwo Backup, CodeTwo Office 365 Migration or CodeTwo Exchange Migration.

To be able to continue with your backup/migration jobs, make sure to assign a Microsoft 365 license that includes Exchange Online to the admin account used in our software. This will automatically enable a mailbox for that user.

Follow the steps below to assign a license in Microsoft 365:

1. Sign in to the Microsoft 365 admin center.
2. In the menu on the left, select Users > Active users (Fig. 1.).

Fig. 1. Accessing Active users page in the Microsoft 365 admin center.

3. Select the user to whom you want to assign the license.
4. Go to the Licenses and Apps tab and select checkboxes next to the licenses that you want to assign (Fig. 2.). Make sure to select at least one plan that includes Exchange Online (e.g. Office 365 E3).

Fig. 2. Assigning an Office 365 license.

5. Click Save changes to apply them.

Once the license is assigned and mailbox enabled, re-configure the connection to Microsoft 365 in the CodeTwo software you are using. Follow the links below for more information:

• connecting to Microsoft 365 in CodeTwo Backup
• connecting to the source Microsoft 365 tenant in CodeTwo migration software
• connecting to the target Microsoft 365 tenant in CodeTwo Office 365 Migration 

You should now be able to resume with your backup or migration tasks. Keep in mind that it may take some time for the changes to fully propagate through the Microsoft 365 tenant. So, in case you still receive the same error, make sure to wait awhile and try re-configuring the connection to Microsoft 365 again.