relay_hosts
Specify the hosts which are allowed to relay email via this ESMTP service. Each item can be an IP literal or a CIDR mask. Note that the CIDR notation is strict, so that 192.168.1.0/24 is valid but 192.168.1.1/24 is not because that final octet isn’t valid in a /24.
The defaults are to allow relaying only from the local host: