You may get this error while trying to update your existing email queues.
The email you entered is the same as your customer email.
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.
To delete the user account, navigate to Staff Control Panel > Users > Manage Users > Search and open the account of the user with that email address and click on Delete.
After deleting the user account, you will then be able to edit and update the Email Queue.