I've had to turn off my newly built eFa

One of our users received a newsletter which had a large number of recipients in the TO: field, she released the email and I noticed the recent messages list was populated with this email over and over again. Each time the subject was prepended with {spam not delivered} and the sender was the system's postmaster email address. By the time I turned this off there must have been 20x {spam not delivered} in the subject.
I looked at the reports and external people/domains were the highest recipients of emails, have we just spammed people with a newsletter? Anyway, keen to know your thoughts...
1. I don't think restarting cleared it out (although I could be wrong), how do I clear this email out?
2. How can I find out if the message was sent externally or caught before it was sent?