Content filtering can add its report as an additional part in a non-mixed multipart message.
Given a message with structure for example
multipart/signed
multipart/alternative
text/plain
text/html
application/pks7-signature
content filtering will append it's text/plain report to the multipart/signed part as in
multipart/signed
text/plain
application/pks7-signature
text/plain
Mozilla Thunderbird and perhaps other MUAs will not render a message with that structure. See https://bugzilla.mozilla.org/show_bug.cgi?id=1353856. While this is really a Thunderbird issue, we shouldn't be adding additional parts to a non-mixed multipart. Instead we should wrap the message as in
multipart/mixed
multipart/signed
text/plain
application/pks7-signature
text/plain