Similarly, when the internal mail server receives a message destined for an external network, it accepts the message from the client and then passes it to a DMZ's SMTP relay. The relay then forwards the message to the destination server. This architecture prevents direct connections from the Internet to the internal mail server, providing a layer of isolation.
As an added bonus, you can use a spam-filtering device as your SMTP relay. Devices like SendMail's Sentrion appliances and the Barracuda spam firewall are popular tools that can reduce the spam-filtering burden on clients.
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