What I've found is that the problem is triggered by having a multi-part message, where one or more parts contains multiple Content-Transfer-Encoding headers...
This behaviour comes from [1,2,3]:
Messages with ambiguous content should be treated as a security risk. In particular, if MIME::Parser is used in an email security tool, ambiguous messages should not be delivered to end-users.
Per default, Amavis delivers them, but flags them as "CC_UNCHECKED,3", which is a dedicated subcategory for this case.
[1] https://gitlab.com/amavis/amavis/-/issues/112 [2] https://metacpan.org/pod/MIME::Entity#ambiguous_content [3] https://metacpan.org/pod/MIME::Parser#ambiguous_content
