The severe mail slowdown for those using pop/smtp today between 11:10-11:30 was caused by myself. I posted to both eskimo-announce and outages-list simultaneously. Both of those lists contain hundreds of local addresses. Each address caused a copy of spamassassin to launch for the delivery of that mail address. This exceeded the memory capacity of the mail server causing it to go into swap and slowdown.
I doubled the amount of memory in the machine and re-tested, after the additional memory, the machine absorbed the list posts without any problem.