bug#34177: smtpmail.el aborts on transient errors

[Brian Sniffen]

smtpmail.el treats all return codes over 400 as errors, and aborts.  But RFC 
821 and successors call the 400-series “transient” errors and suggest retrying 
immediately.  Some mail servers, including current MS Exchange, use 400-series 
errors for load limiting.  The user can just repeat C-c C-c until the mail goes 
through... or Emacs SMTPmail could loop until the server accepts it.

Retrying is a little tricky in the case of multipart commands like LOGIN, but 
in general it’s safe to loop for seconds on any command.

-- 
Brian Sniffen