I thought of that too, there are a couple of services that do this, although not as a browser plugin. I thought it might be more compelling for people to be able to use this service to add this functionality to their application, if they were using a provider like heroku where you don't get a mail server with your hosting.

Thanks for the feedback.

I think it would be better marketed as a tool for initial contact with people you may not necessarily want to have long-term knowledge of where to contact you.

For signing up for stuff, there is mailinator/bugmenot/etc., and many sites block such services anyway.