Outlook 2007 is generating an undeliverable error when user tries to send emails from one of her other accounts.

User has several seperate email accounts in Exchange 2007, all of these emails have been added to her Outlook in the accounts settings.  The user has full access permissions to all of the accounts, when she tries to send mail from any of the accounts except the default account she gets an undeliverable error which states

" You are not allowed to send this message because you are trying to send on behalf of another sender without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission. ".  Funny part is that it states the name of the person that she is sending to, as the person not allowed to send on behalf of another person.

For Example if she tries to send an email to Mary Smith, the message is.

 
Delivery has failed to these recipients or distribution lists:
 
Mary Smith
You are not allowed to send this message because you are trying to send on behalf of another sender without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission.

Hope this makes sense.