If you decrypt / verify a Mail sent from exchange to exchange on the same exchange server it will show it as "OpenPGP" Mail. It will be correctly decrypted / verified.

This is because the Mails are not converted by Outlook to MAPI and as we use the same message class for outgoing S/MIME and OpenPGP Mails the code is confused.
To trigger the error the mail must also be wrapped in MS/TNEF so we do not even see the Protocol header. Only when parsing do we detect S/MIME as we there use gpgme_data_identify to select the protocol.