H_USER Header Flag (V8.11 and Later)
Certain headers are set by the submitting user, such
as Subject:,
whereas others can be added by
sendmail, such as Message-Id:. Those that
were supplied in the submitted message are marked
with this flag so that sendmail
can differentiate them from headers it generated
itself.
No headers have this flag defined by default, and you should never define it in conf.c.
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