I cant be 100% as I dont have it in front of me but...
From the router setup, click Firewall, then General. In here you need to enable the data filter. Then under filter setup, choose data then you need to edit a rule set. It is usually 2.
Once in there it goes through the rules in order so you need to allow port 25 from one ip address and then create a rule after to block port 25 from the rest of the subnet.
Im sure I could go into more detail if needed but hopefully this will point you in the correct direction
