Hey AlternativePhoto,
It looks like you've almost got there, it sounds like you've already clicked on
process bounces in your phpList admin dashboard, and probably have the bounce configurations setup properly. The next step is clicking on
view bounces, and from there you can take a look at why your messages are bouncing back, and then decide to delete that user if you wanted to remove them from your database completely so they didn't just keep on bouncing.
I actually write technical documentation and recently wrapped up my
phpList education channel, I think you would probably get a lot of benefit from reading my article on
handling bounces with phplist. I directly linked you to the 2nd section talking about processing bounces in phpList, if you're having issues getting them viewed following those steps and screenshots, scroll up to the top of the article and read the section about ensuring that phpList is configured to properly handle bounces.
Let me know if that helps you out or not, and I can try to provide you with some other stuff to try if all of that fails.
- Jacob