We've shown you before how to track your outgoing emails, but that can be costly and you might not need to track everything. With Google Analytics you can track emails on a per-message basis completely for free.
In the video above, engineer Amit Agarwal shows how to use this Google Drive spreadsheet script to set an Event tracker on an individual email. The process requires you to have a Google Analytics account (which is free, but requires a bit of set up). Once you're done it's relatively easy to add the tracker to an email, which can be handy for that one really important message you need to know has been seen.
How to Track Gmail Messages with Google Analytics [Digital Inspiration]