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.