Rework upstream SMTP connection handling

[jorgenschaefer]

In an ideal world, we can detect the relay host from OVERRIDE_RECIPIENT if it is set using net.LookupMX. When connecting to the first fails, we can connect to the next. At the same time, we can try and keep a connection open for a while in case we receive multiple messages in one session.

This is tricky, though, as we have to carefully distinguish network errors from mail sending errors.