amavisd-new -- multipart boundary confusion

The Amavis project reports:

Emails which consist of multiple parts (Content-Type: multipart/*)
incorporate boundary information stating at which point one part ends and the
next part begins.
A boundary is announced by an Content-Type header’s boundary parameter. To
our current knowledge, RFC2046 and RFC2045 do not explicitly specify how a
parser should handle multiple boundary parameters that contain conflicting
values. As a result, there is no canonical choice which of the values should or
should not be used for mime part decomposition.