Updating the offline address book can help resolve some fairly common issues with Outlook, notably if someone within the company doesn't appear in your address lists but also if you have additional mailboxes which either don't seem to be available to you or don't disappear when your access to them has been removed.
To manually trigger a full update of your address book:
- In Outlook, go to the Send/Receive menu
- Click Send/Receive Groups
- Click Download Address Book...
- Untick Download changes since last Send/Receive
- Click OK
Outlook will now download a brand new copy of the offline address book