The relaying window contains all options for relaying. These options prevent that your mailserver will be used as a forwarding server. If your mailserver relays unrestricted mail for everyone, there is a big chance your mailserver will end up on a blacklist itself. Once your mailserver is blacklisted large numbers of emailmessages will be denied by other servers. Besided that, once your server is blacklisted the process to remove it from that list again can be very time consuming.
The 'Relay local domains only' checkbox activates the 'Local Domains' list. If this is checked then only mail from and to the listed domains will be possible. Any other mail will be rejected with the warning '550 Pinjo says: not local address [emailadress], not a gateway'. In this way your mailserver cannot be used as a pass-through mailserver, saving you a lot of load.
The 'Relay local Ip numbers only' checkbox activates the 'Local Ip numbers' list. If this is checked then only mail with a from address that matches the 'Local Domains' list and an Ip number matching one of those in the 'Local Ip numbers' list will be relayed. This gives you an even better relaying protection. Note that this window can only be used when the Relay local domains only setting is used.
In this list the domains your mailserver handles should be entered. A match is done between the email-address and the domain list. If you enter a domain like '.com' all email adresses ending with '.com' will be relayed. This is obviously not a good solution. So a bit more specific entering of domains is recommended like 'acme.com' or '@acme.com'
In this list the IP numbers of your own users can be entered. A match is done between the start of the sending IP-number and the (parts of) ip numbers in the list. If you enter the number '194.171.50.' in the list, all ip numbers that start with that number will be allowed. In this case it will be the whole class C-address (126.96.36.199 – 188.8.131.52).
Subnet addressing is not possible yet.
Changes will be activated when pressing the Save button.