Cannot send in Cached Mode

I have a couple of users who experience this problem............

We have an Exchange 2003 Server and Outlook 2003 clients. Most, if not all
are fine, but a couple of clients are unable to send emails (both internal
and external) when Cached Mode is selected. Without Cache Mode they are
fine, but I would prefer them (like 99% of my other clients) to be on Cached
Mode.

What could cause this to happen?

I had a laptop that a previous user was using. I deleted his XP user
account and added the new user as an XP user. Then rebooted the laptop, and
commenced the Outlook Wizard to set up Outlook. But I get the above problem.

What am I doing wrong and how can I resolve this?

Thanks