bug#34177: smtpmail.el aborts on transient errors

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Tue, 09 Jul 2019 20:24:19 +0200
> Cc: 34177@debbugs.gnu.org
> 
> So, indeed, smtpmail SHOULD try resending when given a 4xx response
> code...  but presumably not forever, either.
> 
> It's a bit difficult to test, though.  Does the following patch work for
> you?

IMO, this should at least say in the echo area that it's retrying,
probably with the number of retries.  Bonus points for doing that from
a separate thread or a timer.  smtpmail locks up Emacs too much
already, when the network is busy.