[Exim] smtp auth and outlook express problem

Hi,

I've configured exim 4.10 with smtp auth and it's working fine.
Except sometimes Outlook Express cannot authenticate itself. It's strange.
One Outlook Express 6 can authenticate and another Outlook Express 6 on
another computer cannot. Is there any special bug known?
Exchange Server and other Outlook versions work fine. Testet with several
different computers.

Thanks a lot for any advice.
Michael