The rule set for the Simple Mail Transport Protocol (SMTP) data is shown in
Table 6-9. As shown, the packets governed by this rule are allowed to pass through the firewall, but are all routed to a well-configured SMTP gateway. It is important that e-mail traffic reach your e-mail server and only your e-mail server. Some attackers try to disguise dangerous packets as e-mail traffic to fool a firewall. If such packets can reach only the e-mail server, and the e-mail server has been properly configured, the rest of the network ought to be safe. Note that if the organization allows home access to an internal e-mail server, then it may wish to implement a second, separate server to handle the POP3 protocol that retrieves mail for e-mail clients like Outlook and Thunderbird. This is usually a low-risk operation, especially if e-mail encryption is in place. More challenging is the sending of e-mail using the SMTP protocol, a service attractive to spammers who may seek to hijack an outbound mail server.