2003 Server – Please help! Remote Outlook access

Hi there, firtsly i work for a charity so im not fully skilled at IT but learning as i go.

We have a 2003 server and recently added 5 new users to it. the main point was to enable them to have remote email access through web mail.This was fine and worked a treat. But about 4 days ago it stopped allowing them to log in.it still connects to our server and asks for user and password but doesnt go any further. I can still login in remotely and some of the other older users ive asked still can too.

Does anyone have any ideas on this.
Your help would be very much appreciated.

AL