Encryption breaks when attaching another email-message as a file

Hello everyone,

we have the following scenario:

- User creates a new email (HTML format), the user will then attach another email message as a file that is encrypted and in a HTML format.
- The user then sends the message to a recipient that has PGP as well
- The recipient can open the attached encrypted message just fine, but the actual body does not get decrypted, even though the recipient has the sender's key

We're on 10.3.2 (MP7), Outlook 2013 32-bit in an Exchange environment.

Any help is appreciated!