Error using Outlook 2003 with MS exchange.

I’m setting Outlook 2003 for clients for our newly deployed windows 2003 small business server with Microsoft Exchange.

On one of the clients when I open a new message or click to foreward a message I get the following message:
” The messaging interface has returned an unknown error. If the problem persists restart outlook”

I tried MS Knowledge base but they only have solutions for different clients.

Any ideas would be aprreciated.
Thanks

Canaan