When your email is viewed at its destination, MSGTAG lets you know. You receive an email from MSGTAG telling you that your message has been viewed. The notification also details when the message was sent, when it was viewed, and what the delay has been in between.
The beauty of MSGTAG is that you don't have to change anything about the way you write or send your email. In fact you'll hardly even know it's there — a small icon in your system...