By default Postfix is not an open relay. The parameters mynetworks_style and mynetworks determine what other systems can use your mail server to send messages. The default configuration allows relaying only from other machines that are connected to the same IP subnet as your server. You can limit or broaden the range of addresses that should be allowed to relay by setting the parameter mynetworks_style. If you prefer to limit relaying to the local machine only, set mynetworks_style to "host". You can also set mynetworks_style to "class" to allow relaying by any host within the same class A, B, or C network as your server. For many networks a class setting opens relaying to too many systems. If you aren't familiar with IP address classes, stick to the default "subnet" or more restrictive "host" settings.

Alternatively, you can explicitly indicate the hosts that should be allowed to relay mail by setting mynetworks. If you set mynetworks, the mynetworks_style parameter is ignored. You can list individual IP addresses or specify subnets using the network/netmask notation -- for example, 192.168.100.0/28. This parameter is handy if you need to provide mail relay to hosts outside of your network because you can list specific IP addresses regardless of their relationship to your own subnet. If, for example, you want to provide relaying to remote users, you simply add an IP address to your list. In this case, your remote users need a static IP address, or at least an address assigned from a limited range of addresses. If your remote users do not have static IP addresses, then you have to configure some kind of SMTP authentication.