Renamed Used in Hybrid/On-Prem Environment - Old Email Continues to Receive Emails

To resolve the issue of emails being sent to the old email address of a renamed user in a hybrid Exchange environment, you can follow these steps:

1. Ensure the old email address is retained as an alias: When you rename a user, the old primary email address should still be retained as an alias. This allows emails sent to the old address to be redirected to the new primary email address.
2. Check the email aliases: In the Exchange Admin Center (EAC), verify that the renamed user still has the old email address listed as an alias. If it is not there, you can add it as an alias to the new user's email account.
3. Use PowerShell if necessary: If you need to manage aliases or perform bulk updates, you can use PowerShell commands such as Set-Mailbox to add the old email address as an alias to the new user. For example:

      Set-Mailbox -Identity "NewUser" -EmailAddresses @{Add="oldemail@example.com"}
   

4. Wait for directory synchronization: Ensure that directory synchronization is functioning correctly so that changes made in the on-premises Active Directory reflect in the cloud environment.
5. Inform users: Notify users that emails sent to the old address will still be received by the new user, but they should update their contacts to use the new email address going forward.

By following these steps, you can effectively manage the email flow and ensure that emails sent to the old address are properly redirected to the new user.