When migrating mailboxes, you may encounter the error: “Exceeded message size restrictions.”

This happens when the source mailbox contains emails larger than the maximum message size allowed at the destination server. If the destination mailbox has stricter size restrictions, the migration cannot proceed for those messages.

Cause

  • The maximum message size at the destination server is smaller than the size of mailbox being migrated from the source server.
  • Exchange enforces these message size limits, leading to migration failure for oversized items.

Resolution

To fix this, increase the message size restrictions on the destination server to be greater than the source mailbox size.

Steps to Increase Mailbox Message Size Limit in Exchange Admin Center (EAC):

  1. Login to Exchange Admin Center (EAC).
  2. Go to Recipients.
  3. Under Mailboxes, select the specific user mailbox.

  1. In the left pane, click Mailbox Features.


     
  2. Scroll to Message Size Restrictions and click View details.
  3. Set a higher maximum size (in KB) for messages that can be sent and received by this mailbox. Ensure the limit is greater than the largest message size in the source mailbox.


     
  4. Save the changes and restart the migration.

Additional Notes

  • Always ensure the destination limits are slightly higher than the source to avoid repeated failures.
  • If mailbox content includes very large attachments, verify with organizational policies before setting very high size limits.