In order for all external emails to pass through Postini's anti-spam/virus filtering system, your MX records must look like:

Please make sure your primary mail server IS NOT listed.

Also, please make sure your primary mail server is locked down to only accept incoming messages from the Postini IP ranges and the Webservio IP ranges if you have also purchased Webservio's mail bagging and/or email continuity services.  For more information regarding these settings, please view:  http://faqs.webservio.net/content/25/270/en/what-are-the-postini-and-webservio-ip-ranges-for-my-firewall-settings.html