Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
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Does he have any other verification methods available to confirm his identity, such as a recovery phone number or an Authenticator app, if those were set up previously? If yes, ask him to sign in with his account here to replace the verification methods - https://account.live.com/names/manage
Microsoft always recommends that users have at least three verification methods on their account. This way, if one method is disabled or fails, you can use another method to verify your identity.
If he no longer has any verification methods besides the recovery email address, and that email is inactive, then on the Verify your identity screen, select “I don’t have any of these” and follow the prompts to replace the security information.
Note: If you remove all of your previous security information and replace it with new information, your account will be placed in a restricted state for 30 days.