No mailbox with such GUID - ErrorNonExistentMailbox

Error

No mailbox with such GUID - ErrorNonExistentMailbox

Cause

This error is commonly caused by an unlicensed mailbox account. Check that all of the migrating mailboxes are licensed in Microsoft 365 first.

If a mailbox account has been assigned a license and the error persists, it is usually caused by a non-null value in the "msExchMailboxGuid" attribute of the mailbox account.

Resolution

By default, when synchronizing a user from an on-premises environment with a mailbox assigned, the value of the "msExchMailboxGuid" attribute will also be synced. Assigning a Microsoft 365 license in this case will not result in the creation of a mailbox.

Follow these steps to confirm the issue in Microsoft 365:

1. Log in to the Microsoft 365 tenant using a Global Admin account, and go to the Admin page.
2. Under the Active Users panel, search for the email address of the affected user.
3. Click on the user name, and look under the Mail Settings.
4. The message associated with the mailbox will be :"This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed."

Follow the "Pre-Migration tasks" instructions in the [Azure for MigrationWiz](Filtering objects using Azure Active Direct (AAD) Connect) article. This will allow mailbox to be created in Microsoft 365 so that MigrationWiz can proceed with the migration.