This error indicates that the item being migrated (or its attachment) is too big, and the Exchange server has rejected it.
Resolution:
You can confirm that you have exceeded the server size limits:
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- Sign in to your MigrationWiz account.
- Click on the checkbox next to the mailbox in question.
- On the dashboard, click on the Start button.
- Click on the Start Migration button.
- Click on the red number listed in the Errors column.
- Review the information listed under the migration errors field, and correct it using the appropriate resolution steps specified in the product content for Exchange Server and Microsoft 365 below.
If the average error size is greater than 10MB, you have most likely hit message size limits imposed by Exchange Server.
For more information about message size limits on Exchange Server: