You use one of the following to try to migrate a user's mailbox data from your on-premises messaging environment to Microsoft Office 365:
The New Migration Batch Wizard in the Exchange admin center of Office 365
The E-mail Migration wizard in the Exchange admin center of Office 365
Exchange Online PowerShell
However, you receive an error message that resembles the following:
FailureType : TooManyLargeItemsPermanentException
FailureSide : Target
Message : Error: This mailbox exceeded the maximum number of large items that were specified for this request. --> MapiExceptionMaxSubmissionExceeded: IExchangeFastTransferEx.TransferBuffer failed (hr=0x80004005, ec=1242)
This issue occurs if the user's mailbox contains one or more messages that exceed the 150-megabyte (MB) message limit.
To fix this issue, locate and remove messages that are larger than 150 MB from the user's on-premises mailbox. To do this, follow these steps:
Access the user's on-premises mailbox by using Microsoft Outlook.
Arrange all items in all folders of the user's mailbox by size. Sort the items from largest to smallest so that the largest items are at the top of the list.
Locate the items that are larger than 150 MB. If these items contain important info, save them to another location such as the local computer or a local Personal Folders file (.pst). Then, delete the items that are larger than 150 MB from the user's mailbox.
Return to the Exchange admin center, and then resume the migration.
After the migration is complete, the data can be re-imported to the Exchange Online mailbox.
For more info about how to import items into Outlook in Office 365, go to the following Microsoft websites:
If you experience issues when you move mailboxes to Exchange Online in Office 365, you can run the Troubleshoot Office 365 Mailbox Migration tool. This diagnostic is an automated troubleshooting tool. If you're experiencing a known issue, you receive a message that states what went wrong. The message includes a link to an article that contains the solution. Currently, the tool is supported only in Internet Explorer.