As far as I know, if it's an HTML e-mail and files are being loaded from the sender's web-server, then it's possible for them to track.
This is why Hotmail and Thunderbird disable the loading of "inline" images (meaning, images described in the e-mail's HTML, rather than images that are just attached) by default for addresses not on your contact list.
So... If the e-mail is in trash but it's an HTML e-mail and you're allowing it to load images and other remote files, then it doesn't matter, it would be possible for the sender to track.
And... If the e-mail is in your inbox, but you aren't allowing it to load remote files, then it doesn't matter, you'd be safe to just delete.
As far as I know.