Who's tracking your emails?

This is just plain ridiculous.

In an online eavesdropping case with potentially profound implications, a federal appeals court ruled it was acceptable for a company that offered e-mail service to surreptitiously track its subscribers' messages.

Is it OK for the post office to track who's sending me mail and for what reasons? No. So, what the fuck? Could somebody please explain to me the difference between a post office that has my private mail and an ISP that has my private email?