This error message from google would indicate what your problem is:
Quote:
'The IP you're using to send email is not authorized...'
In order to prevent spam, Gmail refuses mail when the sending IP address does not match the sending domain. To send mail from your server to Gmail, we suggest using the SMTP relay provided by your ISP. Please note that we are unable to whitelist IP addresses or otherwise make exceptions at this time.
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I would guess from that response that your DNS records aren't correct.