What does "UNCHECKED" really mean?

[Damian]

> 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
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