What problem? I don't see how anything you've described causes any problem.
I can access any of my Subsonic sites using:
http:\\myserver.subsonic.org
http:\\External ip:port number
http:\\mydomain.remotewebaccess.com:port number
I can create links in emails using any of the above formats. I can also substitute any of the formats
http://myserver.subsonic.org/share/qGcOAhttp:\\External ip:port number/share/qGcOA
http:\\mydomain.remotewebaccess.com:port number/share/qGcOA
and they all work.
What problem are you having?
If your Subsonic vanity domain isn't resolving correctly from outside your LAN, then you don't have the port forwarding set up correctly or the firewall is blocking, but the firewall shouldn't be the issue if your getting to the site on the host machine from other devices. Go to canyouseeme.org and test to see if the port is open.