Are you trying to get in via DNS resolution, and are you using NAT on your firewall?

Yes and yes.

If so, you can put a hosts file in the appropriate sub-directory for your operating system, pointing the A records to the internal ip addresses.

As I said, this isn't feasible for at least one of the PCs.

Or set up an internal DNS server with the internal IP addresses.

I was hoping to avoid having to do this. It means that I now have to replicate 5 domains internally as well as externally.

Most firewalls don't like to route out and back in.

I noticed. If I use a proper PC, could I configure it to do this properly?