I was seriously shocked to see that the iPhone's native client leaked that bad. 2 questions:
1) Is it possible for an app to leak less?
2) Is the 'leakage' already commonly exploited by firms for specific means? (proving you saw something etc.)
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.
You are correct about iOS. On my iPhone 4 with iOS 5.1, with remote images enabled (by default) it leaks 17 out of 32 categories. With remote images disabled, it leaks 0 out of 32 categories.
In contrast, Outlook for Mac 2011 leaks 2 out of 32 categories (audio and video) with remote images disabled. Mail.app 4.5 leaks 14 out of 32 categories with remote images disabled.
If anyone could help, that'd be awesome!