!
interface
ip address 172.16.x.x 255.255.0.0
ip nat inside
!
interface
ip address x.x.x.x x.x.x.x
ip nat outside
!
access-list 100 permit ip 172.16.0.0 0.0.255.255 any
access-list 100 deny ip any any
access-list 101 permit tcp 172.16.0.0 0.0.255.255 any eq 25
access-list 101 deny ip any any
ip nat pool 1-SMTP netmask 255.255.255.0
ip nat inside source list 100 interface overload
ip nat outside source list 101 pool 1-SMTP