When you run a client application that works with Office 365 operated by 21Vianet such as Lync, Outlook, Windows PowerShell or Microsoft Azure Active Directory Sync tool, your credentials must be authenticated. Microsoft 365 uses a CNAME record to point to the correct authentication endpoint for your location, which ensures rapid authentication response times.
If this CNAME record is missing for your domain, these applications will use a default authentication endpoint in the United States, which means authentication might be slower. If this CNAME record isn’t configured properly; for example, if you have a typo in the Points to address, these applications won’t be able to authenticate.
If Office 365 manages your domain's DNS records,, Microsoft 365 sets up this CNAME record for you.
If you are managing DNS records for your domain at your DNS host, to create this record, you create this record yourself by following the instructions for your DNS host.