Google Networking Best Practices for Large Deployments - Manual (Page 31)

Network Configuration 31
Mail Routing
After you change your MX records to route mail traffic to Google Apps, your email is no longer
delivered to your servers by SMTP. Instead, inbound email is directed to the Google Apps
servers. This essentially eliminates inbound SMTP mail traffic on your network.
Outbound Mail Connections
Depending on your needs, you may have some outbound mail traffic that you wish to send
from your own network, such as automated or batched communications from applications in
your system. You can use Postini Services to route and filter your outbound mail securely. If
sending application mail outbound through Postini is not an option, you can also send mail
through Google Apps as an authenticated Google Apps user over SSL.