Sending meeting invitations to external people -- Outlook 2003

This is the message our staff received when  attempting to sending a lunch meeting to people at work and one person outside of the office:

  "Could not complete the operation.One or more parameter values are not valid outlook meeting invitation"                          

She WAS NOT able to send the meeting invite out -- once she removed the outside person, the meeting invite sent fine.  

I tested this and could send to external people.  Does anyone have any comment why this happened?
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If you have any outlook addins try disabling them.  This may clear up the error.
Hi Below KB article s related to Outlook 98 and above but can work in your case too.;en-us;185049


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