DidTheyReadIt   
by Rampell Software, LLC   

Rated by PC Magazine as a way to "...track the status of your messages more reliably than any major mail client," and featured on CNN, CNBC, USA Today, The New York Times, The International Herald Tribune, and NBC's Today Show, DidTheyReadIt will tell you if, when, and where your messages are read, re-read and even forwarded. It works as a plugin for Microsoft Outlook, a background tool for any SMTP mail account, or as a service for any other e-mail account (Hotmail, AOL, Yahoo, and more), and invisibly adds a tracking code to each message you send. When the message is read, DidTheyReadIt will send you a detailed receipt showing when, where, how long it was read for, and even what kind of computer it was read on.
Compatible with all platforms.