I think we can be pretty sure that - if there's only one "From:" header - then
the MUA won't display an unsigned From: header in preference to a signed one.
No, but, for example, we have no idea whether an MUA will show any of
these things, or something else entirely:
From: Barry Leiba <barryleiba(_at_)computer(_dot_)org>
From: barryleiba(_dot_)mailing(_dot_)lists(_at_)gmail(_dot_)com
From: barryleiba(_dot_)mailing(_dot_)lists(_at_)gmail(_dot_)com on behalf of
Barry Leiba
From: ietf-dkim(_at_)mipassoc(_dot_)org on behalf of Barry Leiba
From: Barry Leiba
From: Leiba
From: Barry
There are more variants of this in heaven and earth, folks, than are
dreamt of in your philosophy. But come; Here, as before, never, so
help you mercy, is it DKIM's responsibility to sort it out.
Barry
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