When you Setup Outlook for Office 365 . The Account Displays wrong SMTP Address. Which was updated to a new one.Even After changing the Primary SMTP From Active Directory is doesn’t update it here.As Outlook was licensed for this old smtp address .
Solution –
Login to Office 365 Portal. Remove the License assigned to the user. Wait for a minute. Assign back the license.
Then you should see the current Primary SMTP address for the account.
Please share precautionary steps once should take while decommission onpremise exchange after migration to office 365