This video deal with issues involving the Microsoft Outlook nickname cache. The nickname cache (or "AutoComplete cache") is used to populate suggested recipient names when you type in the To, CC, and BCC fields in an email.
The nickname cache is updated every time you send an email with Outlook. Issues may occur with the nickname cache when the on-premise mailbox is migrated to Microsoft Online Services.
This video walks us through how to address and resolve nickname cache issues with Microsoft Online Services.