Mac wont connect to 1 particular IP??

I'm having a bit of a problem with an iMac as follows:

It has full internet connectivity and there's no problems anywhere apart from when I try to connect to a particular exchange email server using the servers url or public IP.

The server is on the same network as the iMac and if I try to connect to the OWA by entering the private IP, 192.168.x.x, then it connects fine. If I try to connect to the URL, https://mycompany.co.uk/owa, then nothing, it just times out.

It is getting the correct DNS reply for the public IP, 188.x.x.x, and if I substitute this in the address bar then still nothing.

All the other computers on the network (Mac + Linux + Win + iPhone) work fine with email clients and with the web access OWA. Also access from the internet (other office/home etc) works fine too.

It's just this one iMac and I have no ideas why only the public address will not connect?

Any ideas?

All the other machines are able to access the exchange server both on the external and internal IP and URL.

All the routing is setup correctly. What makes this even more frustrating is that this iMac was able to connect to the server and then just stopped working. Both via the web access and through Outlook 2011??