I guess you have a .20 IP address on the firewall outside interface and the .18 IP address as an additional IP address and you have used port forwarding to forward ports to different LAN IP addresses? In other words the SMTP server doesnt have its own public IP address?
First 2 "object network" configurations define port forwards for connections coming from Internet to the local servers. (HTTPS there just to simulate your other port forwards)
The following 2 "object network/service" configurations are configured to be used in the actual NAT configuration that would in your case NAT the outbound TCP/25/SMTP traffic to the desired public IP address
The last NAT configuration can be considered a default PAT configuration for all the outbound connections that dont have a specific NAT configuration
object network SMTP-SERVER
nat (inside,outside) static 18.104.22.168 service tcp smtp smtp
object network HTTPS-SERVER
nat (inside,outside) static 22.214.171.124 service tcp https https
object network SMTP-SERVER-PUBLIC
object service SMTP
service tcp destination eq smtp
nat (inside,outside) source static SMTP-SERVER SMTP-SERVER-PUBLIC service SMTP SMTP
nat (inside,outside) after-auto source dynamic any interface
I guess you could try your own version of the above. To be honest the actual configuration that does the NAT for outbound SMTP traffic isnt that clear to me either. Should cheat and check the command reference myself.
I'm not 100% sure if the above NAT configuration might conflict with some future configuration in its current form.
Hope this helps
EDIT: If you havent already used, you can use "packet-tracer" command to check whats happening with NAT before and after the configurations. And ofcourse "show xlate" etc.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...