So you purchased Message Discovery? Did you purchase direct from Google or an authorized reseller? MX record changes, are not covered by support, that is your responsibility. MX changes if done correctly, will rarely cause mail flow problems. What will cause mail flow problems is not making the necessary changes in your Google Apps Dashboard, under System - Email. There you will need to enter the outbound Postini settings and add the Postini IP's in the inbound routing, for better spam filtering and to lock down inbound mail flow to only be accepted by Postini's IP. In addition to Postini settings to use GApps, you will need to use the Google Apps Gmail server settings in Delivery Manager.