[ietf-smtp] Survival of arbitrary MIME Content-foo header field?

Howdy.

If a random -- ie, unknown to the handling systems -- MIME Content-foo 
header field is attached to a body-part, how likely is it to survive, 
end2end transit on the net today?
What if it is at the message header level?  Any better/worse?

Tnx.

d/
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net

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