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Readnotify will show you when and where your email message was opened by a recipient (time and IP address). It is a great service when you need to get an IP of a Nigerian scammer which you cannot get by other means, especially if they use Gmail or Hotmail, when you cannot get their IP from headers. Also, many of them wont send emails, wont reply to emails, they only want to chat on YIM. This is when it comes very handy.
It is somewhat useless for Russian scammers though. You can always get their IPs from headers, and Readnotify will not show anything different than what headers show. But for Nigerians it is excellent. Every baiter should have it!