37.5. Configuring Relaying
In the early days of the Internet, mail servers could safely deliver mail to local domains and forward all other email to another MTA, regardless of its source. Today, allowing your server to forward any email that it receives is an invitation for spammers to use your system as a relay. A well-configured server should only accept email for non-local domains from trusted client hosts, such as those on the company network or home LAN. Email sent to local domains is safe, and can be accepted from anywhere.
For this reason, the Sendmail packages that come with modern Linux distributions are configured by default to prevent the server accepting non-local email from anywhere excpt the same system. If you are setting up a ...
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