How to Lock Down Your Firewall
To prevent spam and viruses from circumventing the message security service, it’s important to lock down the firewall for each of your email servers. This means limiting SMTP (port 25) traffic at the firewall to allow connections only from Postini IP addresses.
Some virus and spam senders specifically target mail servers using low-priority DNS MX records or by looking up a server directly using a common naming convention such as mail.yourdomain.com. To prevent malicious senders from bypassing the message security service, we highly recommend that you add all of your domains to the service, then configure each of your email servers to accept mail only from Postini.