Google‘s mail service Gmail support multi-mail management in your account. Say that you have a primary mail account as firstname.lastname@example.org and added email@example.com to this Gmail account. If you were sending e-mail from the e-mail address with another domain (e.g. firstname.lastname@example.org), the recipient will see “from email@example.com on behalf of firstname.lastname@example.org” 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.