When we have already had a Windows Domain and am Office 365 tenancy, how can we combine user names so that the user can start using their Offie 365 resources with their Windows Domain Usernames?

How do we correct the synchronisation so as not to corrupt the domain user or Office.com user? 🤦

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If you make sure that the SMTP login on Office 365 matches the login for your Domain login - i.e. you use the .suffix that your Office 365 logins use on your domain, the sync tool will look for duplicates and try to match them on first sync.

You can read about it here:

use smtp matching to match on premises user accounts to office 365 

active directory aadconnect to existing tenant 

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