I wanted to post in here since you also opened a ticket with us to assist with this issue. I wanted to get the information out on the community in case someone else wants to setup this scenario.
In order for each domain to have it's own banner response for example '220 mail.domain1.com' you would need a public IP for each separate banner response due to how the hostnames are mapped within SmarterMail and bound to each IP address.
Since you're behind a NAT, you would need multiple WAN IP's to perform a 1:1 mapping for the IP hostnames.
Regardless of banner response, your users across your multiple domains can authenticate to any IP you have listening. So users from domain1.com, domain2.com, domain3.com could all authenticate when connecting to either internal IP address configured within SmarterMail.
I hope this helps !
Technical Support Supervisor