I posted about 6 months ago with an issue of emails being bounced back with a Blank error...
"This is an automatically generated Delivery Status Notification.
Delivery to the following recipients failed permanently:
Reason: A message that you sent to the following recipient could not be delivered due to a permanent error. ** The remote server ?? responded with: ** email@example.com ??:?? This message was created automatically by mail delivery software on the server ."
This is now happening more often and in some cases, emails are being sent with no bounce back but the emails aren't being received. In all honesty, it's driving us nuts as sometimes it can take 3-4 attempts at sending an email!