You may get this error while trying to update your existing email queues. As a part of security update (v4.80.2) we fixed a security vulnerability which can cause the potential compromise of an email queue set up in Kayako.
Due to this fix, Kayako does not allow you to add the same email address as an email queue and as an end user.
To fix this, you are requested to delete the user account associated with this email. You can do that from:
Staff Control Panel > Users > Manage Users > Search and open the user account of the user with that email address and click on Delete.
You will then be able to edit and update the Email Queue.