Hello,
Hoping someone can provide a suggestion on best practice and any iRedMail configuration changes that could facilitate.

We have an existing mail server and domain, and a new domain and new iRedMail server setup for it (LDAP backend). We have only 1 internet facing IP.

We need to keep the old domain and server running for some time, while the new iRedMail is up and running, until we reach a point we can fully retire the old one.

Which creates a problem since we can only port forward to one address, and domain is not at the tcp layer of course.

The topology currently looks a bit like this:
domainA.com mx -> 1.2.3.4:25
domainB.com mx -> 1.2.3.4:25

WAN 1.2.3.4:25 ->port forward-> LAN 192.168.0.10:25

mail servers:
    internal-domainA-server:25 (old mail domain/server)
    internal-domainB-server:25 (new mail domain/server, iredmail)

currently: 192.168.0.10:25 is internal-domainA-server.

New domainB is not live on the internet (for receiving)... yet.

For most internally hosted services we are able to reverse proxy with nginx to work around the issues of only a single internet facing IP. Purchasing another public IP is not option for us.

What would be some solution options to be able to do something similar with the mail? I am thinking a mail relay might be the answer.
And if that is the case...
With iRedMail (not pro, I wish I could afford it, maybe someday there will be an smaller SOHO option), would it be possible to add to our new iredmail server configuration a second domain (of our legacy domain), and configure that to only relay those inbound (from internet) messages on to the proper server (internal-domainA-server)?  And thus enable us to put the new iRedMail server into the firewalls port forwarding configuration replacing the old one. (We want to get rid of the old domain, and don't want to migrate it and all the mail/accounts/etc).

Or if iRedMail non pro cannot be setup this way in a simple manner, it is not a big deal for us to load another vm, is there a suggested setup which would do some smtp relay and then split and relay each domains incoming internet mail to the appropriate mail server?

Or if this is not the right route, any suggestions are greatly appreciated.
Thanks!