It can happen fairly easily, that one or more user’s Office 365 login is wrong, and AD Sync fails to correct it. Here is a manual override, courtesy of the excellent Joe Busby.

1. On a domain controller, verify the AD username and proxy addresses is set correctly.
2. Connect to O365 Powershell on any machine with Powershell 5.1, and run this command:

Set-MsolUserPrincipalName -UserPrincipalName "user@old.address" -NewUserPrincipalName "user@new.address"

3. On the user’s PC:
   1. Navigate to Settings > Accounts > Access work or school
   2. Disconnect from any accounts linked to the new or old username
   3. Hit the [ + ] Connect button and provide the credentials to sign in to Office 365
   4. If their password has been changed since their account was synced up with the wrong username, the password may need to be reset.
   5. Remove any credentials from windows credential manager that match the old username.
   6. Check Outlook, you may have to create a new profile.