For some users the MAPI to MIME conversion in GpgOL is not working. After sending the mail in set looks like it has the PGP Message as the body and it's decrypted like its the body.
As a result the MIME structure is printed into the body and not properly parsed resulted in a broken display, especially if attachments are used. (Our parser does not expect headers for PGP Messages).
The user that showed me the problem uses Kopano 8.3.4-12 through Exchange Active sync.
As I doubt that we can influence the sending we might be able to do some heuristic when reading such a mail e.g. we switch into MIME mode if we see a mime structure in the decrypted / verified content.
A test mail is:
The PGP Message in there decrypts to a valid mime structure.