This is pretty easy to do. In the server manager, add in the ip addresses that you want to allow to relay. These IP addresses will be added to the local networks section of the server manager. Any ip addresses added here will be allowed to access any of the services that can be accessed on the LAN side of the server. Make sure that the server is in "Server and Gateway" mode and you should have no trouble. You must know, though, that these IP addresses will be able to access everything that you can on the LAN, ie. server-manager, FTP if it is set to private, etc...
Also, if you want to specify only one ip address, use the subnet 255.255.255.255.
Hope this helps