The easiest way to fix this is to create an alias for the external URL and effectively "punch a hole in the local DNS-server."
To do that, go to http://hda/setup?sub=aliases&tab=network
* Go to Setup > Settings and turn on advanced settings
* Go to Setup > Networking > Alias
* Add a new alias
* Fill in alias: the service you want to reach on your external URL, but only the first part, e.g. "mail" or "www"