bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 51733@debbugs.gnu.org
> Date: Wed, 19 Jan 2022 14:06:56 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> There's no scanning -- rmail displays the From header, right?  So it
> >> does decoding before displaying the header.  It has to do the textsec
> >> stuff first, too.
> >
> > Not if textsec is optional, it doesn't.
> 
> I don't understand what you mean here.  rmail will call
> (decorate-suspicious-email from) and then insert the result into the
> buffer.  If textsec is switched off, it'll just return `from' as is.

But From is not the only place where a suspicious address could hide.
It could also be in the body, or in the quotation parts.  We cannot
rely on header decoding alone to do this job well.