Can not send to email recipient from MS Word.

When a user tries to send mail from within a Microsoft Word document with an attachment she gets the following error:
xxxxx@xxx.xxx on 10/1/2008 9:29 AM
You do not have permission to send to this recipient. For assistance, contact your system administrator.
MSEXCH:MSExchangeIS:/DC=int/DC=domain:MARVIN.

What can be causing this to happen? The meassage goes fine when sent through Outlook