We are planning to add true multi-port hosting like we have already with our FTP server to the mail server. However, for now you can only listen on one port. As Phil said, the easiest way is to run IIS or other web server on port 8000 and redirect to AMS on port 423 or if your router can forward multiple ports to a single port on AMS internally then that would be easier. However, if you're not already hosting any web server on that Internet IP, it would be much better to just use https default port 80 then you can access via
http://mail.site.com without any port specified.