bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  51733@debbugs.gnu.org
> Date: Wed, 19 Jan 2022 13:56:33 +0100
> 
> Robert Pluim <rpluim@gmail.com> writes:
> 
> > (textsec-email-suspicious-p "דגבאa <foo@bar.com>")
> 
> That is not a valid email address.

??? My INBOX is full of mail from people with such "invalid"
addresses.

What is not valid about it?