Interal Access to External IP Address

My mail server sits on my network and has an IP address 100.100.100.5; I am using a Netgeat DG834GT ADSL router with port forwarding and I can access my mail server from outwith the network on IP Address 212.159.xxx.xxx.

My users inbound and outbound mail servers within Outlook are set to 100.100.100.5 - all works great. When we are out the office we can access the mail server on 212.159.xxx.xxx. Again all great.

The problem I have is that I would like to set the mail server in Outlook to the WAN IP address of 212.159.xxx.xxx so that some new users with laptops have seamless access to email wherever they are.

However I can not get the laptops to hook up to 212.159.xxx.xxx when they are in the office; it only works when they are attached outwith the company network.

I know it is possible as I have set up an almost identical system elsewhere that works perfectly.

Can anyone offer any advise on where I should be looking to resolve the problem.

Thanks.