I don't believe this is a feature already (if it is, I just couldn't find it).

We regularly send out mass mailings to our students which contain images hosted on our domain. I would like for these images to be accessible without clicking the "Display external images" button, but I would like to avoid enabling "Display External Images" globally.

Is it possible (or could a feature be easily implemented) to allow the display of external images only from a trusted domain?