The company's "How it works" page explains the system to some degree; it involves redirecting all mail to be tracked through their servers by appending "didtheyreadit.com" to your recipient's email address. I doubt this is mutt-compatible ... Reader xrxzzy points out USAToday's article on the service as well.
iosdaemon writes "didtheyreadit.com claims to be able to track your sent email: "When, exactly, your email was opened. How long your email remained opened. Where, geographically, your email was viewed. DidTheyReadIt works with every single internet provider and e-mail account, including EarthLink, AOL, NetZero, Juno, Netscape, Hotmail, Yahoo, and much more." Read on for more."This appears to be snake oil. I put it to test just in case someone had come up with some magical code. I sent email from a Yahoo.com account through the service, to an account on a Linux Box. Running tcpdump, I received the email from my pop and let 5 minutes pass before opening it. I left the message open with the cursor in the text for another 5 minutes. Tcpdump revealed absolutely no questionable traffic. And, the service control panel indicated the email had not been viewed. Sending email to a Yahoo.com account results in a 'read' in the service CP. But I had the message open for 10 minutes, and it indicated a 2-minute read......"