If you are having difficulties sending to a known address within your organisation and have already cleared the auto-complete cache in Outlook, you may need to manually remove and update the Offline Address Book:
- If Outlook is open, close it.
- Browse to the Offline Address Book at C:\Users\[your computer username]\AppData\Local\Microsoft\Outlook\Offline Address Books
- There will be folder(s) at this location labelled with a lengthy string of characters. Empty these folder(s) of their contents, but do not delete the folder(s) themselves.
- Launch Outlook.
- Select the "Send/Receive" tab, click "Send/Receive Groups", and select "Download Address Book".