There's no 100% reliable way to do this unless you have complete control over the software used to read e-mails by the people you send messages to.
CodeProject is your friend: "How to know whether an email is delivered and read:"
"A Read-receipt is returned from the receiver to the sender when the email is read successfully by the recipient. Microsoft Outlook employs this mechanism but not all the email applications supports this, not only that a recipient can disable sending receipts altogether. Because of these factors, Read-receipts are not a completely reliable way of knowing the delivery of emails. The Read-receipt can be implemented by inserting one or more of the following lines into the email headers:"
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