Google has reported that mail sent outbound via Postini to Proofpoint hosted domains is not being delivered as of 11:23 am EST. Messages may be deferring with a 451: cannot connect to domain:<recipient domain> - psmtp" error code or bouncing with "550 MX records inaccessible for too long for domain: <recipient domain> - psmtp".
This appears to be an issue with the recipient domains' DNS servers, but Google engineers are investigating.
Update: Issue is resolved. Mail should now delivery correctly to the domains that are hosted on Proofpoint.