[viraptor]

I'd accept it if it had only false negatives in "different person" detection. False positives are not really acceptable with this method now that we've got people using webmail/clients/mobiles.

For example, my client at work is on all the time - it will happily pre-cache all the images. But the same message will be visible on my mobile too - what happens when I open it there? If I got an error, I'd probably check with website for accessing the same mailbox - what would happen after 2 "forwards" - image deleted?

This is a reality of today's offices, not an edge case I'm afraid. It will also fail for shared email accounts (for example "info" or "support" type destinations). Or auto-forward while someone is on long vacation will do exactly the wrong thing - let someone else read it, but not the real recipient.