Question : Outlook 2003 calendar invitations get bounce back

When one user sends a message for a meeting with his staff he keeps getting a bounce back as follows:

The following recipient(s) could not be reached:

      Name on Date  Time
            The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.

The following information might be helpful:
1) We're running on active directory, and using Microsoft Exchange Server
2) We're not enabling delegation
3) The employees who have this problem are the employees who run two accounts on the outlook 2003, one of these accounts is POP and the other is Exchange.

Thanks alot.

Answer : Outlook 2003 calendar invitations get bounce back

Make sure they're sending it from their Exchange account. Also, have them click on the To button and manually select the recipients from the Global Address List, to see if there's something buggered up with their contacts.