Msg Loged in ApplicationLog regarding IM

Plz any one can sort out this prob. the message is “Teh
requesting principal
hhttp://mailservername/instmsg/aliases/username could not
be matched with the authenticated user. (Error 1).

Although the user is loged on and can send message
through instatn messageing.

I have contact Technet but it says the case is
“you receive this error message as a warning message
because you are attempting
to log on to Internet Messaging with valid but
inappropriate credentials. For
example, you are using the credentials of another user.

STATUS
======

Microsoft has confirmed this to be a problem in Microsoft
Exchange 2000 Server.”

but i do not understand what is the main case it is would
u plz help me