This awesome little plug in for Apple Mail, MailTrackerBlocker hosted at Github is a must install. If you don’t know, companies install something called a tracker pixel into the html of an e-mail they send you, and when it loads the image off their server they can track your location and often more information. These spy pixels should be illegal, but they aren’t so do what you can to stop them!
Honestly I would think Apple would do something about them, and especially on the iPhone and iPad. They are so into not letting companies track, then they should stop this insidious behavior. At least you can do something about it on the desktop.
It is scary how many e-mails have these built in trackers, which you can see once you install this app. Basically every e-mail with html formatting has one.