bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 51733@debbugs.gnu.org
> Date: Tue, 18 Jan 2022 13:47:35 +0100
> 
> I think the places it would make sense to hook this machinery in would
> be in:
> 
> * shr (displaying URLs and links)
> * Gnus/rmail (displaying email addresses)
> * Message (when responding to mail; a prompt "do you really?")
> * browse-url (prompt)

Sounds reasonable.

Perhaps also Tramp (host names)?

> There should probably be a customization point?  A user option like
> `warn-about-suspicious-identifiers'?

Is this a go/no-go test, or are there levels?  If there are levels,
perhaps something similar to NSM would be more appropriate?  (And
maybe levels of NSM should determine the default textsec level?)