For people whose email addresses' domains also have a website, you could peek into the site and display images from the site like happens on Facebook when posting a link from a page.

I understand this is magic, but it seems like you could do this well below a certain popularity of domain (after the top million?).

Also, and perhaps more reliably, you could look at URLs in the sig of the message.

This would give more ways to see what the person looks like online overall, not just social networks. This would be more helpful than social networks for small business in many cases.