Appears to be an easy task at the first glance, enabling DHCP relay on a Sophos firewall can be a little bit “tricky”.
First thing, you can’t add multiple DHCP servers. That’s a big issue because if your only server become down, you lost DHCP for all vlan behind your firewall. You can try to create an object group with both servers, but you will not be able to add it ont the config page.
Another strange thing too, you must add both server interface and user interface. Yes Sophos is unable to bind by itself the interface the DHCP server belongs.