A shared mailbox recently had about 5 e-mails coming in per minute. Some were the deliverable notifications, and some were bounces from the external China mail servers. In a day it reached almost 30, 000 e-mails.
I still don't know what caused this in my case but I thought i'd share what I did to fix it.
I first did my due diligence by scanning the possible infected computers which found nothing.
The next step was to reset the password for the e-mail account. This seemed to have slowed it down but still not solved.
I then went in the queue viewer where I saw the e-mails were continuously looping. I removed the queue and the problem has been resolved.
I apologize I didn't have screenshots, I wanted to fix the problem, go home, relax and watch TV.
Hope this helps!
Kyle