Beginning 3:33 pm EST, Postini was notified of errors for customers on System 8. The error being reported is "550 MX record points to an invalid IP for domain"
Postini is aware of this issue and they are currently investigating the cause and working on getting a resolution for this as soon as possible.
Remember that you can monitor the following sites for updates for this issue and others that Google and Postini are working on.
3/13/2014 2:00 pm EST Update: Postini is reporting that the issue is resolved. If you are still seeing this issue please let us know.