Edit Office 365 users to change email addresses to a different domain

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Office 365 operated by 21Vianet - Admin More...Less

You can’t remove a domain from Office 365 operated by 21Vianet if there are users who still have email addresses that use that domain.

For example, if you want to remove the fourthcoffee.com domain from Microsoft 365, you must first change all user email addresses that end with@fourthcoffee.com so that they use a different domain.

Sign in to Microsoft 365 with your work or school account.
Select the app launcher icon in the upper-left and choose Admin.
Go to the area where you edit users, and then change email addresses to use a domain name that is not the one you’re removing.