People might not arrive at your website outside of Office 365 operated by 21Vianet if they include “www” when they type the site address in a browser under certain setup conditions. This happens if you have a domain that you've added to Microsoft 365 and you've set up the domain so Microsoft 365 manages your domain's DNS.
For example, if your domain is fourthcoffee.com, after you set up your domain and redirection to your website from Office 365, people can get to your website by typing http://fourthcoffee.com in a browser. But they won't get to your website when they type http://www.fourthcoffee.com. You probably want both addresses to resolve to your website. You can make sure this works by adding "www" redirection.
Do you manage your domain's DNS yourself outside of Microsoft 365 at your DNS host? In that case, you can set up redirection at your DNS hosting provider.
Set up “www” redirection for the website with your domain
Many hosting providers offer URL redirection services. At your DNS host’s website, search for “redirection” (also called “301 redirection”), and follow the steps there to set up forwarding to http://www.yourdomain.com, where yourdomain.com is your domain name.
If your DNS host doesn’t offer redirection, you can use another company that provides URL redirection services. There are a number of companies that offer these services. To set up redirection, you buy an Internet link from the company, and the link redirects people to your website by using the fully qualified domain name (that includes the "www") for the website.
Why do you need to set up redirection?
If Office 365 manages your DNS recordsand you have a website outside Microsoft 365, you can set up redirection to the website so people can still get to your site after you add your domain. You use an A record (and perhaps a CNAME record) to do this. Then when people go to, for example, http://fourthcoffee.com, they can still see your website that’s hosted outside Microsoft 365, even after your domain is on Microsoft 365.
But even after that’s set up, if someone types the URL for your website that includes “www” instead of just the domain name, they won’t see your site. You can fix this by setting up “www” redirection. Then, for example, if people type either http://fourthcoffee.com or http://www.fourthcoffee.com, they’ll get to your website outside Microsoft 365.