A server we look after was the victim of an NDR attack and / or open SMTP relay attack today. Since around 8:30am this morning, none of the recipients have been able to receive email from external sources (mail sent internally is fine). I've completed the steps explained at:-
Now, the queues no longer fill with NDR's, but it still hasn't resolved the problem where email is not being received. I'm sending emails to the recipients connected to this Server, but the emails aren't reaching their inboxes.