3D Mailbox: Email Tracking and User Status
See the online/away/offline status of anyone you email, and see the status of every email you send: from the time it leaves your PC to the time the recipient replies -- including if it gets caught in the Spam filter! (Provided your friends also use 3D Mailbox).
This is a revolutionary new way to make sure your emails make it to their destination, don't get marked as spam, and don't "fall through the cracks". Unlike that annoying "return receipt" feature you've seen in other email programs, this is unobtrusive, intuitive, and completely under your control (you can choose whether others can see you and/or track their emails to you).