outlook 2007 0x80040115 Exchange 2003 https/rpc

Hello Techgurus,

I have a Windows XP SP3 laptop that is running Outlook 2007. The user of
this laptop is unable to send emails. Get following error message:

"Task Microsoft Exchnage Server reported eror (0x80040115): The conection to
the Microsoft Exchange Server is unavailable. Outlook must be online or
conected to complete this action"

Outlook connect to exchange server through https/rpc

Outlook 2007 establishes connection fine ( takes while) and then it looses every few second. and the user can
receive emails fine.

I have tried the following troubleshooting:
- Rebooted the computer; the problem still exists
- recreated outlook profile -  problem still exists
- The user can log in to OWA and everything works fine there
- 3 out of 6 user connecting to the same Exchange server exchange 2003 has this problem

Please help!

Many thanks