If you are having problems sending an email to a certain person or to a distribution list like PBS All, HS Teachers, MS Teachers, etc, then you may need to clear your auto complete list in Outlook using these steps:
Open Outlook and click "File" and then click "Options"
In the Outlook Options window that appears, make ensure that "Mail" is selected on the left-side of the window and then scroll down using the scroll-bar on the right-hand side of the window until you get to the "Send messages" area.
Click the "Empty Auto-Complete List" button and then click "Yes" in the dialog box that appears.
You have successfully cleared your Auto-Complete list now and it will begin to repopulate email addresses as you compose and send messages to people.