Knowledge Base

Troubleshooting the ‘Mailbox does not exist’ error


You are no longer able to start backup or migration jobs in CodeTwo software or perform other actions that involve connecting with your 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.


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 an Office 365 license that includes Exchange Online to the admin account used in our software. This will automatically enable a mailbox for that user.


If you are using CodeTwo Office 365 Migration to migrate data between two Office 365 tenants, remember that both admin accounts need to be mailbox-enabled: the one that connects with the source tenant, and the one used to configure a connection to the target Office 365 tenant.

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

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

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

  1. Select the user to whom you want to assign the license.
  2. 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).

Assigning an Office 365 license.
Fig. 2. Assigning an Office 365 license.


If necessary, you can also navigate to and expand the Apps menu and select the checkbox next to Exchange Online only (Fig. 3.).

Enabling the Exchange Online plan only.
Fig. 3. Enabling the Exchange Online plan only.

  1. Click Save changes to apply them.

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

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 Office 365 tenant. So, in case you still receive the same error, make sure to wait awhile and try re-configuring the connection to Office 365 again.

Was this information useful?