Messages stuck in Outbox not visbile

Hi All,

I have a strange, but interesting issue with a series of clients in our
environment.

These machines are using Windows XP + SP3 (including latest patches), with
Outlook 2003 + SP3 (including latest patches).

The issue at hand is that some users are finding that some messages are
getting stuck in their outbox, but are not actually able to see these. All
users in the environment are using Cached Exchange Mode, and if these users
log on via OWA, they are able to see these messages which are stuck and force
them to be sent. The problem is that they are unaware messages are stuck
there until it is too late.
We can turn off Cached Exchange Mode and see the messages, but the users
which are affected currently travel a fair bit and we would prefer to keep
this setting enabled.

We are running Exchange Server 2003 + SP2 (with the latest patches).

We've already attempted the following;
Rename the OST, and recreate
Create a new mail profile
Create a new user profile on the local machine (the users don't have a
roaming profile)
Delete the account from AD, including exchange data and recreate the
account/mail./
The users have used different machines and still experience the problem
The users machines have been rebuilt (formatted and reinstalled)

Any suggestions welcomed and thanks in advance.

R