Actually that's pretty much how I got started in the domain business. There was a company offering to handle email for free on your domain, which you could either sell or place ads on. The company later went under. Everyone.net started doing it shortly after that, but only offered 50% of banner space. I never made any money off that either, and discontinued trying vanity email on my domains.
I had thought of doing it myself, but you have to worry about your domain getting blacklisted if a spammer starts using it. Great idea, but there are a lot of hurdles to implementation.
Also, note the netidentity.com has offered the vanity email service for a long time. They may be hard to compete with if they also own a version of the name you have.