Why should Hotmail, Yahoo! Mail, and others, be the only ones earning money from footer ads? The user should make some money too — and it’d be fine if there was 1 more ad in the footer — maybe even a visual banner ad (a la RSS feeds).
The real question is how to accomplish this, and that is likely why it has never happened. You might need a browser plugin that would see when you send off an email — and you’d likely have a black-list of recipients that you don’t want to receive the ad in the footer.
This brings me back to my one idea about user-approved advertising (which Facebook has done now in a sense with Beacon). These ads in the footer of your email don’t have to be meaningless from you (the sender), they could be things (sites you use; products you’ve purchased) in your footer. They could be your commerce-related “tweets” from Twitter; or from your Facebook newsfeed.
Don’t forget about Outlook — this might be an easier plugin to use.
But plugins are a barrier for most people to use your service. So it might be that the service providers themselves (Hotmail, Gmail, etc) would have to give you this functionality…. unless you have another idea?
As always, all of these ideas are here to stimulate other ideas. Use the comments section to give more ideas for this idea — and by all means, if you feel one of these ideas is worthy of your precious time, then go on an execute on any one of them.