This document describes how to reduce the impact of spam with empty message bodies.
What can I do about spam with no subject or message body?
While Cisco Anti-Spam creates and publishes rules to combat spam attacks that appear like bounce messages (blank sender), there is a message filter which can help reduce the impact.
if ((subject == "^$") AND (header("To") == "^$")) AND (body-size < 3072)
insert-header("X-Spam", "$FilterName"); quarantine("Policy");