Is this a home computer or one that you used connected to Microsoft Exchange in your Office?
If you do connect to Microsoft Exchange and the person you are sending to is part of the Exchange contact list and you are trying to send when not connected to Exchange, then this is a known problem.
It relates to Outlook not being able to resolve the address of the recipient when you are not connected to Exchange.
If this does sound like your issue, then please reply and I will dig out the Microsoft Knowledge Base article for you. If you are not an Exchange user then none of this will be in the least bit interesting to you
