Notes on Relaying

One of the most recent additions to Sendmail is protection against spam (unsolicited e-mail) through anti-relaying rules. These rules have been available for a long time, but only recently—as of Sendmail version 8.9—has the default configuration been to disallow relaying of messages from one server to another.

For a legitimate dial-up or remote user to use your SMTP server to send a message to another remote recipient, your server has to act as a relay, forwarding the message on to the recipient even if the message didn't originate from and wasn't addressed to anyone on its machine. Functionally, as illustrated in Figure 25.2, this is exactly how a spammer would send an unsolicited message to the same recipient through the same ...

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