My understanding (I don't have an iOS device) is that the standard iOS email client has remote images enabled by default, and if you turn that off, then it doesn't leak at all. I think all email clients should have the option to automatically load remote images, but that it should be disabled by default. The problem is, users don't understand that by clicking the "load remote images" button, it will be possible to track them, so they often don't understand its purpose.

It is possible for an email app to not leak at all. All it has to do is not load remote content unless the user specifically requests it.

Yes, these tricks are commonly exploited. Most of the time, people just use a simple img tag pointing to a 1 pixel image, although recently there was some news about Facebook using the bgsound tag.

I don't know if law enforcement ever use these sort of techniques to try and track down suspects, but it wouldn't surprise me.

Interesting that it seems to loads remote Flash content. I wonder if you could do anything nefarious with that capability. I don't know Flash enough to comment.

Does it have an option to enable/disable automatic loading of remote images? If so, what is it set to by default? And does it still leak if you disable loading of remote images?