[Ignitionnet]

Think you misread the OP, it's nothing to do with port forwarding, if it were we'd be needing port forwarding to browse websites. Port forwarding is only an 'issue' with incoming connections, and there's no reference to 'internal' or otherwise beyond that it works fine over Orange Broadband but apparently not over Virgin Media. Outlook Anywhere is no different from browsing an HTTP(s) site beyond what's carried inside the HTTP(s) payload. Unless OP is running an Exchange server on his Orange DSL there's no inbound / outbound and OP is not hosting the Exchange on his DSL, he just mentions it working ok via Orange.

Port forwarding is not needed internally whether on default or non-default ports. If internal to the server then you aren't going to be going through a firewall / NAT gateway to be port translated but routed which is why port forwarding is a non-issue internally.

Remember that port forwarding is there to overcome dynamic NAT - port forwarding makes the NAT static so that it's a 1:1 relationship between port and machine - come in on port x you will always go to machine y, otherwise you would best case go to a DMZ machine, or most likely the packet would just be dropped as the firewall has no entry in its' state table for the connection.

James - are you using a VPN to access Outlook Anywhere or is it a normal HTTPS / SSL session?

I'm asking as Virgin Media doesn't seem to like VPNs too much - they seem to have issues with fragmented packets.

Also what MTU do you have set on your routers and your PCs?