The email specifications don't include such a check as a guaranteed feature. You can ask the user to tell you they have read it. Most will ignore such a prompt. In many cases, a mail server along the way may delete such a request, so the user never sees it. This is called a Read Receipt. '''Options|Return Receipt''' in Thunderbird's Write window Alternatively there is a DSN (Delivery Service Notification) which asks servers to tell you when a message has been processed. So at best it could tell you the message was delivered to a mailbox, but not that it has been read. Most mail servers have this feature disabled; it is seen as a tool for spammers to detect good and bad email addresses. '''Options|Delivery Service Notification''' in Thunderbird's Write window. There are independent services that relay your email message and add a "beacon" to it. This is typically a tiny (1 pixel) image that when downloaded and viewed registers this event with the image hosting server, which can then tell you the image has been looked at, and by inference, your message was viewed (but possibly consigned straight to the Trash.) Some users might be offended to discover you were spying on them in this way. Canny users might use tools or techniques for blocking such a "web bug". In Thunderbird you might view the message in plain text, or view it offline, or use "phone home" detection add-ons. So, in short, no. There is no way to know for sure what has happened to your email after sending it.