ReadNotify is an extremely sophisticated certified email service that has options to let you know whether an email you sent was opened under all circumstances and provides proof of your sending, too, while hiding most of the complexity behind easily accessible plug-ins, tools and email shortcuts.

Pros

ReadNotify lets you know (even under all circumstances) whether an email you sent has been opened You can prove you sent a message with ReadNotify or make mail self-destruct, unprintable ReadNotify offers an easy to use Windows tool as well as email shortcuts

Cons

ReadNotify does not allow recipients to opt out of "invisible" receipts Messages with restricted recipient access interfere with normal email workflow You cannot edit certified ReadNotify messages after they have been sent