If you operate an email server on the Internet, you have a responsibility to make sure that you do not create an open relay that spammers can use as a launching point for their activities. An open relay is a mail system that permits outside systems to send mail to other outside systems, passing the messages along so that the originating system does not have to deliver directly to its target. Spammers constantly scan for misconfigured systems that permit them to relay mail. Before spam became such a problem on the Internet, mail administrators often operated open relays because it made their systems convenient for their users. Now nearly all SMTP software systems are configured by default not to be open relays. Postfix is no exception.
If your system is abused as an open relay, it will most likely be so bogged down with sending spam that its performance will be hindered for your legitimate users. If you choose to accept spam into your own system that is, of course, up to you, but you must take steps to ensure that your system is not used to abuse other systems. There is a good possibility that if spammers use your system to relay mail, your network will end up on a blacklist. Once your site is blacklisted, many sites will reject all messages from your network, both relayed spam and legitimate messages from your users. Chapter 4 discusses safely configuring Postfix to prevent your system from being abused.