Google's mail service Gmail support multi-mail management in your account. Say that you have a primary mail account as yourname@gmail.com and added yourname@yourdomain.com to this Gmail account. If you were sending e-mail from the e-mail address with another domain (e.g. yourname@yourdomain.com), the recipient will see "from yourname@gmail.com on behalf of yourname@yourdomain.com" at message header.

Now, Gmail removed this ugly feature, and won't include this messages anymore. Happy for Gmail users however they should do this a long time ago.