...
When using the How to bulk change users email address there are situations where the target email has already been used. For example, a re-org happened, and users created their email addresses. This will result in the change not being possible. However, you can force this change by ensuring that you’ve selected the force indicator during your scheduled task.
...
By doing Doing the above , this lets the app create a dummy email address, force forcing the change and send the dummy email address for deletion.. If there are many domains in this situation, you might consider performing How to bulk delete users instead