The problem is that we replace the encrypted text and attachments with the decrypted / verified parts. This would already be a modification even without such changes like the category.

I wonder though if it is only some changes that are problematic? We do most just in the outlook object model and try to avoid sync back to MAPI as we do not want to have the plain text synced to the exchange server. But we do some changes to MAPI, like changing the message class and setting some flags for us. If only these would be the problem we might be able to avoid them. Although the message class change is very hard to avoid. The code for the modification is mostly in the parsingDone function https://dev.gnupg.org/source/gpgol/browse/master/src/mail.cpp$1876

But replacing the mail contents itself is very deep in GpgOL, reworking it to work on something like temporary copies would be a major refactoring.

we replace the encrypted text and attachments with the decrypted / verified parts

That's what I feared, but it seemed like a security issue to do that as the mailboxes could be shared to others.

I guess most users of this addin are using IMAP accounts and it's fine there : only the cached copy inside the mapi profile is updated but as messages are immutable in the IMAP store, the "save" function is a noop there.