I'm building a web application, call it http://mywebapp.com, where a user should be able to create a newsletter and email it to everyone in his user account's contact list. So he can check mark all his friends, select an html template, fill in the body content, and send.

The only catch to this is that after the user sends the email, the recipient must NOT be allowed to trace the source of the email back to http://mywebapp.com.

The reason for this condition is because the user is a "reseller" of my business who wants to appear as if he's an independent business. If liability issues arise due to the reseller's unduly usage, I do not want to be caught up in the legal issues. Hence, I do not want anything about my business or technology to be traceable back to me by newsletter recipients.

What's the best way to achieve this?