off of your inside interface, then you will need to add a static route to it and the NAT config would look something like this:

ciscoasa(config)# object network nat-range1
ciscoasa(config-network-object)# range 203.98.225.20 203.98.225.40
ciscoasa(config-network-object)# exit
ciscoasa(config)# object network inside-net
ciscoasa(config-network-object)# subnet 10.10.10.0 255.255.255.0
ciscoasa(config-network-object)# nat (inside,outside) dynamic nat-range1 interface

ciscoasa(config)# object network SMTP_SERVER
ciscoasa(config-network-object)# host 10.10.10.16
ciscoasa(config-network-object)# nat (inside,outside) static 203.98.225.9 service tcp smtp smtp

If the 10.10.10.0 subnet is going to be located on one of the other interfaces, replace "inside" with the name you assign to that interface.