I checked my System Trends --> Traffic Reports and am seeing astounding usage for such a limited # of domains and email accounts. Looks like in the last week, my lowest mail volume was 108,000 messages and highest was 495,000 messages.

I then dug into the System Summary Reports --> Traffic Reports and was able to see that the obscene mail volume came from one domain (1,321,582 messages). Drilling into that, I found the one account that appears to have been compromised sending all but 34 of those messages.

I went into the domain manager, picked that domain to manage and checked the user account. Sure enough, the password was simple accountname123. I changed its password and marked it as disabled with no access to mail services.

...

At the end of this, my spool still had a good 100,000 messages. I wrote a script in VBScript that looked for a handful of common strings that were found in the spam messages (either the compromised account, or various sender/recipients) and was able to clear it out with a few passes of the script. My spool is empty, my CPU utilization isn't crazy high (on a dual CPU machine) and my memory utilization for MailService.exe is hovering right around 160mb.