bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 51733@debbugs.gnu.org
> Date: Wed, 19 Jan 2022 14:16:48 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > 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.
> 
> The scope of the relevant implemented functions are to determine if the
> (on-wire) mail headers are suspicious or not, and do so reliably.  We
> can add a slew of other functions for other types of DWIM
> suspiciousness, of course, but that's outside the remit.

I disagree with this narrow definition of the scope.  textsec is more
general, and should not limit itself to specific wire protocols.

I'm not asking to _replace_ RFC2047 support, I'm saying that we should
also support email addresses that were already decoded, for the use
cases where that could be more convenient or where the wire level is
unavailable.  Why would you object to extending these functions so
that they could support decoded email addresses?  What harm could that
possibly do?