Outlook 2003 fails to connect to Exchnage server 2003

Pulling my hair out a bit, need some help. In our office we have 14 clients all running win XP and Outlook 2003, on Monday morning we all lost our connection to exchange server, most lost it when the sending mail some when they restarted their machines.

Exchange appears to be running alright, have managed to send a mail from the server account and when sending inward mail I get a message from the postmaster on the domain.

When you open outlook, in the bottom right had it says disconnected, thats it. Has to be exchange end as all client cannot connect.

Am I missing something blatantly obvious, could anybody help me with a quick 5 step guide or so of things to check please.

Any help appreciated

Graham