GpgOL is sometimes modifying email messages at the mailbox on its own.
There is an external process sending plaintext messages with an inline OpenPGP signature to a mailbox hosted in an Exchange server.
Alice connects to said mailbox natively (MAPI/HTTP?) using Microsoft Outlook, where GpgOL is installed.
Bob connects to the same mailbox via IMAP using thunderbird, with no PGP plugin.
Bob would be able to view all email messages exactly as if Microsoft Outlook client was not involved at all.
When Bob accesses the emaiks, some of the messages became multipart/mixed, and now show an attachment identified as GpgOL_original_OpenPGP_message.txt:
Content-Type: text/plain; name="GpgOL_original_OpenPGP_message.txt" Content-Description: GpgOL_original_OpenPGP_message.txt Content-Disposition: attachment; filename="GpgOL_original_OpenPGP_message.txt"; size=...
(interestingly, the first and second MIME parts -which both show as base64- contain exactly the same content, except that the first one has a new line prepended, which is probably wrong)
Not all of the emails opened by Alice change in the mailbox. Probably depending on some timing, the "decrypted" version of the signed email, which would be prepared for display, get saved into the mailbox, thus affecting other clients.