It is expecting that account to exist locally. So it appears that you have the local set to yourdomain.com. There is a link in the proposed solution that can be followed to explain address classes. The problem you may be having is that by mapping your domain, you are forcing postfix to use valid local accounts that exist. I am unsure in Postfix how to do local and relay domain for the same domain. It appears that you will want to set postfix to see your domain as a relay domain and not a local domain. See the following link: