Well, first of all you wouldn't use a
servlet. Servlets process HTTP requests, but e-mail has nothing to do with HTTP.
What you do is this:
1. Set up an e-mail server that can receive e-mails at mydomain.com.
2. Make sure it's set up to reject as much spam as possible.
3. Program it so that when it receives a message for
abc@mydomain.com, it finds out what real address that corresponds to and resends a copy of the message (presumably including the advertising that pays for your site) to that real address.