The other day when using the Microsoft Online Transporter to migrate Exchange mailboxes to Microsoft Exchange Online, the following error was being returned on several of the mailboxes:

(401) Unauthorized

Typically when you see an error like that, you assume it’s a permissions issue. In this case, the strange thing was that the mailbox migration had actually begun copying data to the Microsoft Exchange Online Mailbox and failed in the middle of the migration. After reviewing the migration logs, there were numerous warnings about timeout issues. To get around the issues, we simply reduced the number of simultaneous migrations, and reran the migrations. In other cases we just reran the migration and/or waited until off hours to retry the migration.

Was this article helpful?