Sometimes email can get stuck sending in the Outbox, usually if it's a large file, but sometimes it just becomes corrupt, and it can bring Outlook to a halt because no other email will go out until this is removed.
- Open Outlook and select the Outlook menu in the top left-hand corner to the left of File.
- Select Work Offline from the menu.
- Right-click the Outlook application icon on the Dock and select Quit from the menu that appears. It may not, and if it doesn't, right-click again and keep doing it until you see Force Quite on the menu and click that.
- Re-open Outlook.
- On the left-hand side of the Outlook window, you should see something that says "Outbox."
- Select Outbox and delete the unwanted email messages that have been stuck here.
- Go back up to the Outlook menu and uncheck Work Offline -- the checkmark should no longer appear next to this item.
- You should now be able to send and receive email.