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Invalid characters in UPN and email address
Hi, Quite a large number of mailboxes are not sync to Cloud because special characters are not supported in Office 365. Is invalid character support in: - public folders where email address has...
You'll need to clean that up or prevent the sync of those objects by filtering in AADConnect
here is some info on what is allowed:
and the tool that can help identify and fix these:
I did quite a bit of testing on this, there are some nuances and you can add back addresses with special characters as a secondary alias and that will work. See https://cloudrun.co.uk/exchange-online/special-characters-in-upn-and-email-addresses-in-office-365-migrations/ for more info.