 <p>There aren't any guaranteed reliable ways to do this. There are a number of methods that work some of the time, but there's no way to be certain. Most of the methods that could have been reliable are routinely blocked by end users due to spam.</p> <p>The most common way is to send an HTML email with graphics that are loaded from your site (or quite frequently from a third-party tracking agency's site). The graphic would be loaded and the URL would be spiked with a unique ID so you know which recipient has loaded it.</p> <p>However this only works if the user (a) reads their email in HTML mode, (b) allows it to load graphics, and (c) reads it while they're online.</p> <p>Some techniques use Javascript to perform a similar task. But that has all the same issues, and can also be stopped by users blocking Javascript in their email.</p> <p>The best method (ie the most socially acceptable one, and least likely to be blocked) is to provide a link for the user to click on to get more info, which has a unique ID. This of course doesn't tell you what's been read, but it does tell you who's interested in what they've read, which is probably more valuable to know anyway.</p> <p>The down side of all these methods is the need to give each user a unique ID. This means that each email you send has to be unique, which means quite a big processing overhead for your mail system as it has to re-generate the text for every single user. This is the reason that most people who do this sort of thing delegate the task to a third party tracking agency.</p>