I've done a few of these over the years all using the Exchange admin centre. They've all been easy enough and I've never had any problems until now.

I have an IMAP mailbox with 10 years of history and 40-odd thousand emails. The smaller accounts have all worked as expected but this one returns an error I can't troubleshoot.

Error: ExchangeDataException: Decoding of header Subject failed; raw value: =?shift_jisx0213?B?SW1wb3J0YW50IHNlY3VyaXR5IG1lc3NhZ2UgU01UUC00?= --> Character set name (shift_jisx0213) is invalid or not installed.

The problem I'm having is in trying to work out which email is causing the problem. Admin center shows me nothing but the error with the report really just telling me how far through it gets.

If I try using Get-MigrationUserStatistics in Powershell, it also shows nothing. The reports are empty when I query Subject, Sender, etc for a given Identity.

BadItemLimit appears to be depreciated with the batch automatically handling errors but the import stops immediately rather than skipping. I can use ApproveSkippedItems and restart the batch but that only appears to skip the one record which has broken the import restarting the batch sees it fail again.

I assume I'm missing something obvious with either identifying the records or skipping them altogether, anyone have any suggestions?