Problem Description

You are unable to start migration jobs or perform actions that require connecting to your Microsoft 365 (Office 365) tenant using Stellar Migrator for Exchange.

The software displays one of the following (or similar) error messages:

  • Mailbox does not exist

This error prevents successful authentication and migration operations.

Root Cause

The Microsoft 365 admin account used to configure or run migration jobs in Stellar Migrator for Exchange is not mailbox-enabled.

A mailbox-enabled admin account is a mandatory requirement for:

  • Connecting to a Microsoft 365 tenant
  • Creating and running migration jobs
  • Performing tenant-to-tenant (Office 365) migrations

If the admin user does not have an Exchange Online mailbox, the connection to Microsoft 365 fails and results in this error.

Solution

Assign a Microsoft 365 license that includes Exchange Online to the admin account used in Stellar Migrator for Exchange. Assigning such a license automatically creates and enables a mailbox for the user.

Important
If you are performing Microsoft 365 tenant-to-tenant migrations, both admin accounts must be mailbox-enabled:

  • Source tenant admin account
  • Target tenant admin account

How to Assign an Exchange Online License in Microsoft 365

Follow the steps below to assign a license to the admin account:

Step 1: Sign in to Microsoft 365 Admin Center

  • Go to the Microsoft 365 admin center and sign in with global admin credentials.

Step 2: Open Active Users

  • From the left-hand menu, select: Users > Active users



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

Step 3: Select the Admin User

  • Click the user account that is used with Stellar Migrator for Exchange.

Step 4: Assign License

  • Navigate to the Licenses and apps tab.
  • Select at least one license that includes Exchange Online, for example:
    • Microsoft 365 E3
    • Office 365 E3
    • Any plan that contains Exchange Online




(Fig. 2: Assigning an Office 365 license)

Optional: Enable Exchange Online Only

  • Expand the Apps section under the selected license.
  • Ensure that Exchange Online is checked.(Gig:3 below)







(Fig. 4: Enabling the Exchange Online plan only)

Step 5: Save Changes

  • Click Save changes to apply the license.

Post-License Assignment Steps

Once the license is assigned:

  1. Allow some time for Microsoft 365 to provision the mailbox (this may take several minutes).
  2. Open Stellar Migrator for Exchange.
  3. Reconfigure the Microsoft 365 connection using the same admin account.
  4. Retry the migration job.

Additional Notes

  • License and mailbox provisioning may take time to fully propagate across the Microsoft 365 tenant.
  • If the error persists immediately after assigning the license, wait a few minutes and try reconnecting again.

Outcome

After assigning an Exchange Online-enabled license and reconfiguring the connection:

  • The ‘Mailbox does not exist’ error should be resolved.
  • You should be able to successfully connect to Microsoft 365.
  • Migration jobs can be resumed without interruption.