For no other reason than a link posted by ewx, I counted up the spam trapped by my filter over the last 3 weeks and the last 7 days with their rejection reasons.
Those are scarily high figures. :-(
|Last 3 weeks|
|No From: line
Spam that got through in the last 3 weeks: 72
of which those during the last week: 39
[Recent modifications to the filter would now have junked 31 of those 72.]
Figures of which rule junked each message are of course biased by the order in which they are checked in the filter, which is: Bcc, No From:, DNSBL, rogue domain, rogue user, HTML, base64, windows-1251, SpamAssassin, spam score (with earlier matches taking precedence).