Some Outlook users may experience the following:

• Free/busy lookups fail on conference rooms and users.
• Email is bounced back to the sender with a message stating that the user cannot be found.

This is caused by the Outlook cache referencing addresses that are no longer valid even if the entry appears to display the correct address. These addresses can be stored using internal legacy addresses that are no longer valid in Office 365. To resolve this issue, one of the following must be done:

• On a case-by-case basis, start a new email, type the address until the cached address appears. Delete the address from the Outlook cached addresses using the delete checkbox on the right of the entry (see below). Once deleted, re-enter the correct address and send again.
  
• Outlook for Windows - Delete all cached addresses from Outlook. This will remove all cached entries and they will be re-saved over time as they are re-entered. To clear all items in the cache, select Options from the file menu in Outlook. In the Mail section of the Outlook Options page, click the Empty Auto-Complete List button.
   
• Clear the cache in Outlook for Mac