While looking at T7242: GpgOL: Shown attachments duplicate on selection of signed emails an inital confusion shows up in the display and error messages of GpgOL that it does an S/MIME validation. This means that the placeholder string for decryption and verification for s/mime is used. Since internally we correctly switch over to OpenPGP after collecting the data and using gpgme_data_identify on it the state of the Mail stays confused. e.g. if the error is key missing the Signature button will ask you to look for the certificate in the configured x500 directory services. So this might lead to more problems down the road if it initally takes the wrong protocol.

This is with latest Outlook 17726 and Exchange 15.20.7828.0

Using Outlook spy we can see that the pgp/mime mail is wrapped inside a container named smime.p7m. The mail was initally sent from KMail. My suspicion is that any multipart/signed is wrapped up to be "opaque" signed so that modifications during transport do not break the signature, but while this is the basis for PGP/MIME support in GpgOL at all. I cannot remember that it went so far as to be a "virtual" smime.p7m attachment.