Can NOT send emails to a person from Office Outlook, but can from Outlook Web App

Exchange server email accounts

Can NOT send emails to a person from Office Outlook, but can from Outlook Web App

Cause: Outlook saves a cache (offline) address book. It seems to take one or two days to update the cache. After the update, we can send emails to the person from Outlook.

Solution: We can not wait one or two days before sending emails to the person. Click Tools | Account Settings | Change. Clear the check mark in front of 'Use Cached Exchange Mode'. Re-start Outlook.