CiviMail keeps delivering (and bouncing) if SMTP server disappears

Happens with CiviNewsletter delivery via CiviSMTP and has been reported on the forums.

The connection is lost (or is corrupt) and we get an invalid sender message with a timeout. So the process keeps going with a bounce every 2 mins. We should treat this similar to the socket error of sendgrid: i.e. dont record a bounce and abort the process if it happens 5 times in a row