Is this via a return receipt?
The common way to do this (it looks like SuiteCRM does the same) is by adding a small image to the email. When the image is requested with a given id that email is marked as viewed. This means that only those clients that accept images are marked as viewed however.
Viewing the message source, there is a 1px x 1px image appended to the end.
Its a tracking pixel!
Check this out if you want to learn how to build one!