MSGTAG 

MSGTAG notifies you the moment your emails are opened.

When your email is read, MSGTAG lets you know. You receive an email notification from MSGTAG telling you when your email was opened. The email notification also tells you when it was sent, when the email was delivered, and what the delay was in between.

Unlike traditional read receipts, MSGTAG doesn't hassle your recipient with pop-up boxes, it's completely automatic and you can get read receipts from practically any address.

The MSGTAG email tracking software works with almost all desktop email programs (including Outlook, Outlook Express, Thunderbird, Windows Live Mail, Incredimail, Eudora and Pegasus Mail).