A few hosts add additional MIME content (advertising, footers), but the ones
I've paid any attention to seem to do so by
fairly crude textual insertion, so probably wouldn't corrupt existing content
too much.
To the extent that the unknown MIME header field depends on the
associated content to be unmodified, that's an obvious problem.
The ones I've seen look at the MIME type enough to know whether to add
text or to rewrite HTML, so they probably would not mess with a
mystery part.
However my impression is that such additions by mediators, such as
mailing lists or anti-abuse enterprise front-ends, are done to the
primary body-part -- the cover note -- and not to any attachments.
Or they splice on a new part and wrap it all in multipart/related.
I'd think a mystery type at the top level would be likely to trigger
spam and malware filters that not altogether unreasonably assume that
something mysterious is likely to be evil.
R's,
John
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