Whither X-Image-Url?
Hey, did anyone else notice that Apple has silently dropped support in Mail.app for the X-Image-Url header? It would fetch a small graphic from the given URL to display in the header of an email, a feature inherited from NeXTSTEP’s Mail.app. Yeah, it’s something of a security hole, but it was a fun feature. I had been intending to exploit it by stuffing some kind of graphical message meta-data in there; of course, when I tried it, Mail.app ignored the URL I supplied.
Maybe inline graphic data is the only safe option—perhaps the Face header proposal, or some kind of specially tagged MIME part.
Mail.app will still show icons in email messages, but only if there’s a picture associated with the email address in your Address Book. (example above: Mail.app uses my iChat icon)