Offboarding mailboxes fails with “PropTagToPropertyDefinitionConversionException.”

Hi, a migration failing at a consistent percentage with a property conversion error usually means the move is hitting a mailbox item or property that Exchange cannot translate cleanly during the move.

I would start with:

1. Run `Get-MoveRequestStatistics -IncludeReport` and look for the exact bad property or folder.

2. Try the move with a small bad item limit only if your policy allows it.

3. Test with one affected mailbox and one known-good mailbox to see if it is mailbox-specific.

4. Check whether the issue started only after the Exchange SE upgrade.

5. Make sure both Exchange Online and on-prem endpoints are fully updated and healthy.

If multiple mailboxes fail with the same property error after the upgrade, I would open a Microsoft case with the move report attached.