​​404 Not Found

This error indicates one of the following situations:

1. The specified credentials are invalid.
2. The specified email address for the mailbox is not the primary SMTP address.
3. The admin account has insufficient permissions to log in to the end user's mailbox.
4. If Office 365 is the Source or Destination endpoint, "404 not found" can indicate that the Exchange module is not active for the associated tenant. This is often due to the wrong type of license having been assigned.  Note: Office 365 Business SKU licenses do not include Exchange. 

This error may also occur in rare cases if an item was deleted while the migration process was enumerating the item within a folder.