Rebooting should have killed the mail queue.
In nDepth, can you do a search for "InternalRuleFired.ExtraneousEmail = *email*"? That should show you everything that fired that triggered an e-mail. Now, click on one of the events in the nDepth results and pick the "Explore --> Event" option.
You'll get a timeline like this:
That let's you see the event(s) that triggered the rule that sent the e-mail. Have you resolved whatever changed that spammed e-mail or modified the rule if this is the new norm?