1. Send the same message to the intended recipient from OWA. If the message is sent and received without issue, then it is a local issue with Outlook.
The problem lies within one of these areas:
2. Clear the Auto Name Resolution. ( File > Options > Mail > In the Send Messages section, click on the Empty Auto-Complete list
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 of what has been shared, removing and re-configuring your account again.