This is Gmail's explanation of the new "via" information shown on incoming emails: https://support.google.com/mail/bin/answer.py?hl=en&answer=1311182&ctx=mail.
Since you are indeed sending emails from a 3rd party, there is no way to change this without breaking regulations. The actual sender of the email must be identified in the header information, and if google is going to pull and present this information, there is no way to alter it without misrepresenting the actual mail server. This will be the same coming from any autoresponder.
We are developing a way to provide clients virtual email servers, so check your Inbox for developments.
[Article last update 12/28/11]