One out of the box option here would be to target corporate email addies, as Outlook allows for graphical based signatures.

Set up your own business-y website, and use @yourdomain.com emails sent through Outlook to other @company.com emails. Of course this is also hoping that employees shop Ebay from their work computers, but you get the idea.

There are always backdoors... we need to stop asking "Is it possible?" and start asking "Is it worth it?" The solutions always exist, they just aren't always worth the effort.

Personally I've found that business in general, and marketing specifically is all about mindset... shift your mode of thinking and you'll be in a better position every single time.