Zendio is one of those things you want for yourself but hope no one else has. For $9.95 a month, this Outlook add-on will tell you whether and when someone has opened your email message. It will also tell you whether he or she has clicked on any of the links included in your message, and where the recipient is located geographically. All without alerting the recipient that this info is being collected.
That makes Zendio more reliable than the sending receipts available in Outlook and other email clients. The receiver of the email has to either set the software to automatically send receipts or individually approve a receipt before it’s sent. Either way, getting a receipt tells you a message has been read, but not getting one doesn’t tell you anything.
Read more at CNET.