This article assumes that it is possible to determine if the mail has been read. For the methods to do this see <a href="https://en.wikipedia.org/wiki/Email_tracking" rel="nofollow">https://en.wikipedia.org/wiki/Email_tracking</a>.<p>For a real plain-text email you can only use Read-Receipts but several mail readers don't support this and even for those that support it the default setting is probably "Off".<p>That leaves only the creepy use of tracking images (e.g. the famous single pixel gif), and most privacy conscious users will not load these images.