When installed, by default, Postfix is configured to accept mail locally for users existing in your machine's domain. Postfix's default security settings also stop your machine from acting as an open relay.

j r - - r An open relay is a mail server that allows any user to send mail through your mail ' - ~ ■ server, regardless of his or her location (in your local network or the Internet). This has proven to be a large contributor to the existence of spam as malicious users can use your mail server to send large quantities of mail on the back of your network bandwidth.

The built-in security features of Postfix have contributed to its success, and these security features can be extended even further by adding granularity for unsolicited commercial e-mail (UCE) and user authentication for relay controls.

Postfix is commonly configured in the following three main ways to provide MTA services to your organization. We discuss these throughout the chapter:

■ Always-on, Internet facing mail server.

■ Dial-up/laptop mail server mainly used for sending mail from your machine only.

■ Local mail delivery only. This is used to mainly route mail messages that originate from your local machine and is the default for a SUSE installation.

Local mail delivery is also capable of connecting to other Simple Mail Transport Protocol (SMTP) servers if mail is sent using the Postfix mail system. We discuss this later in the chapter.

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