3G broadband dongle – cannot log in to exchange server for emails?

Hello

I am using an Orange 3G dongle (orange are the phone company) to access the internet on my laptop. I can surf the net fine but when I try and log into my Email using microsoft exchange server 2003, the windo comes up asking for username and password but when i put them in, it just repeats the same window asking and doesnt actually log me in?

can anybody help please?

many thanks

alex