User tracking can be used to tell whether a unique recipient of an HTML-formatted message has opened the message in their e-mail client. phpList tracks user activity by embedding a tiny (1 x 1 pixel) transparent image into each HTML-formatted e-mail that it sends. This image, instead of being an actual image file, is a unique URL to a phpList script, whose function when loaded is to record a "hit" in a database table against a unique message ID.
phpList can then examine that database table to tell which messages destined for which users have been opened.
The embedding tracking image will look something like this in the source of your HTML message:
<img src="http://url-to-your-phplist-install.com/ut.php?u=1a555c58f885bf66ec8571d217eea18d&m=26" width="1" height="1" border="0">
As this technique relies on the e-mail being displayed in HTML, it won't work on users who receive your messages in plain text format.
This common technique for e-mail opening tracking suffers...