I noticed a signigicant drop since Friday. Did some checking...
Comcast started blocking port 25 a while ago, and about a week ago, took additional measures to prevent spam from being generated on their subscribers local machines.
These measures may impact Comcast subscribers who are users of Web Wiz software who are running SMTP on their local machines. According to the article linked below, there appears to be some workaround available.
About 10% of all worldwide spam originates on the Comcast network. Some people estimate that 15-25% of all US spam originates there.
http://zdnet.com.com/2100-1104-5230615.html