If you are having an issue with sending an email to an address that you are certain is valid (i.e. it works from an outside address like Gmail or Yahoo), here is a great way to determine whether or not it is a local issue on your computer.

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.