Hi Keith,
It sounds to me like your IIS site is not really running on port 50012, but instead your firewall acts as a proxy server and routes requests from port 50012 to an internal machine with a site running on port 80. Maybe the current site runs on port 80 so you are trying to run the mojoportal site on a different port from the firewall to keep the official site working while you work on the new mojoportal version of the site. Just guessing here.
This would work as long as all the internal links are relative but in a few places we use fully rooted urls starting with http for internal links and since the web server is really running on port 80 it does not correctly build links with port 50012 because from its perspective it runs on port 80 and it doesn't know anything about the proxy server.
To me the best solution would be to setup an alternate host name like dev.yourdomain.com and point it to your firewall ip address and route both the official domain and the dev host name to the same internal IIS server with 2 sites configured on port 80 and using the same internal ip address but differentiated by the host headers so that the requests go to the correct site.
Alternatively you could configure your dev site to actually run on port 50012 so that it is not mapping across ports from the firewall/proxy server and the internal links will be built correctly with the port 50012
Hope that helps,
Joe