From Rampell Software:
DidTheyReadIt is an e-mail service that tells you if, when, where (geographically), and for how long your e-mails are read. With the DidTheyReadIt Outlook Plugin, each e-mail that you send from Outlook has a tracking code added to it automatically. When the message is opened or forwarded, you receive a detailed report indicating when, where, and for how long it was read, providing much more data and more reliability than standard e-mail receipts. Useful for tracking success rates of sales pitches or important e-mail communication. With the DidTheyReadIt Plugin, all e-mail is sent normally through your own mail server.