OWA connection Problems

I have set up an exchange server at home with a IP addres resolved to my domain i.e mail.mydomain.com, so when I try to connect remotely using mail.mydomain.com/exchange I connot connect. I get a page cannot be displayed error, cannot find server orDNS error. The strange thing is this is from my office however other people in different locations, using the same URL can connect, also using a dial up service from my office I can conncect to Outlook but cannot see any of my mail.
Please Help, DoI have a configuration prob or fire wall prob
Thanks