sudo echo "127.0.0.1 didtheyreadit.com" >> /etc/hosts
Also, the way this service works by appending an invisible image into the originator's email. The image pulls from didtheyreadit.com, so you could also filter on didtheyreadit.com in the message (i.e. set up a new rule looking for that string). It would be nice to reply to someone asking what the problem is...
[robg adds: From what I've read, the service isn't as good as it claims to be, but it does indeed allow basic email tracking. This is the technique spammers have used for years, it's just been commercialized and publicized by didtheyreadit.com. Another effective workaround is to disable image loading in your email client; in Mail, that's in Preferences -> Viewing, then disable 'Display images and embedded objects in HTML messages.']

