The fix for T4525 has a side effect in that it required a change to the message class to mark S/MIME mails with attachments as "no s/mime mails" so that Outlook does not try to reencrypt the modifications.

With the changed message class we are back to the incompatibility fixed by using the standard message classes in Gpg4win-3.1.8.

A fix for this could be to let the "move" pass and only after this change it back to the standard value.