To direct all the incoming email for your domain to Office 365 operated by 21Vianet, you have to create a specific mail exchange record (MX record) at your domain's DNS hosting service. The value you add is different for every domain. You can find the value to add by signing in to Office 365 and looking at the DNS records list for your domain.
We provide step-by-step instructions for creating MX records and other DNS records at many popular DNS hosting providers.
If you have more than one MX record for your domain, make sure the MX record used for email flow to Office 365 has the highest priority.