Error sending meeting requests
 

I have a user ther gets return messages whenever he tries to send a meeting
request. This appears to be happing for both internal and external contacts.
I've seen this issue posted around the Internet, but no solutions. I also
saw a this link on MS KB, but I don't see how it applies to Outlook:
http://support.microsoft.com/kb/823917/en-us

The error message that is bounced back looks like this:

' on 9/11/2006 3:00 PM
This message could not be sent. Try sending the message again
later, or contact your network administrator. Error is
[0x80070057-00000000-00000000].

' on 9/11/2006 3:00 PM
This message could not be sent. Try sending the message again
later, or contact your network administrator. Error is
[0x80070057-00000000-00000000].

Internal, User on 9/11/2006 3:00 PM
This message could not be sent. Try sending the message again
later, or contact your network administrator. Error is
[0x80070057-00000000-00000000].

'External Contact )' on 9/11/2006 3:00 PM
This message could not be sent. Try sending the message again
later, or contact your network administrator. Error is
[0x80070057-00000000-00000000].

Any help would be greatly appreciated.