Do not quit the SMTP sever on 421 error code

smtplib is already closing the connection when such error code is received.

Closes #13710
(grafted from e91426cb5d9db204250399823ab9e2acba640b1f)

--HG--
branch : 7.0

trytond/trytond/sendmail.py

@@ -55,7 +55,8 @@ def sendmail(from_addr, to_addrs, msg, server=None, strict=False):
             logger.error('fail to send email', exc_info=True)
         except smtplib.SMTPResponseException as e:
             if count and 400 <= e.smtp_code <= 499 and hasattr(server, 'uri'):
-                server.quit()
+                if e.smtp_code != 421:
+                    server.quit()
                 server = get_smtp_server(server.uri, strict=strict)
                 if server:
                     continue