No email client can do this. Users cannot be forced to acknowledge receipt, and the server-based features, in the rare cases when they are enabled, only tell you that the message has been accepted by a server, not that the message has been downloaded or opened. I think the most effective way to track email is to embed a link to an image into it. Such links are known as webbugs. When the message is opened, it downloads the image from the server and the server knows the message has been opened. I suspect that the link can be made unique so the sender knows who has looked at his message. This technique can be circumvented by a savvy user. He might only read messages offline, or use a text-only mode to prevent links and images being rendered. Thunderbird offers this capability, to the ''recipient'', by disabling remote content in untrusted messages and thereby blocking the download of the embedded image. I don't like these covert methods. It seems somewhat underhanded to me.