In many networks, the email server, running via SMTP, exists internally to the protected network, usually on a demilitarized zone (DMZ). In these cases, it is common to place Anti-Spam between the outside world and the email server, usually on or as close to the firewall or gateway as is possible.
In such situations, it is common practice for the domain name servers (DNS) to have their mail exchange (MX) records for the appropriate domain to contain the IP address of the firewall. The firewall then uses a system of network translations to direct incoming email to the local SMTP server.
In these scenarios, it is usually not necessary to make any changes to the existing DNS records to direct mail through Anti-Spam. Anti-Spam would be configured to relay mail from the firewall to the internal SMTP server.
For assistance in setting up the external DNS servers, see External Self-Managed SMTP Email Server.