If you are unable to send messages to an email address that you are certain is valid (i.e. it works when you send from an outside address, such as yahoo or gmail), here is a great way to identify whether or not it is only a local issue on your computer ... and how to solve it.
1. Send the "same" message to the intended recipient from the OWA portal. If the message is sent and received without issue, then we immediately know that this is a local issue that resides in Outlook.
2. Clear the Auto Name Resolution.
3. You may have the same contact email address in both the GAL (Global Address List) and a local Address Book. If you share any other address books in Outlook, try unsharing them.
4. If you are unsure about what has been shared, we recommend running the New Profile Connector Tool again to reset everything back to factory defaults.