Can’t add email address after Tenant Migration with this message: This email address has already been added

Situation: The client is in processing to migrate their Micrsoft Tenant. During the migration, they create a temporary email address for forwarding. After the migration, they try to switch to the original email address, but get this message: This email address has already been added.

Troubleshooting 1: Delete the old email profile and create a new email profile.

Troubleshooting 2: If the above doesn’t fix the problem, run registry editor (Regedit) to delete the email address.

Troubleshooting 3: Run Credential manager to remove old email credential.

Troubleshooting 4: In some cases, if you have Windows update pending, restart the computer.

Troubleshooting 5: If the device was joined to Azure DC, you may want to create a local ser account.  Note: Don’t remove the device from Azure AD except you are sure. If you do want to remove the device from the Azure AD, make backup for example, Documents, Desktop and Favorite folder.