When a new mailman subscriber is created with the email of an already existing mailman subscriber or user a `409` code with a message `user already exists` is returned by the postorius but when the same email but now case sensitive is used a `500` code with message `server error code` is returned.
The latter should also return a `409` instead of `500` as it is misleading.
When a new mailman subscriber is created with the email of an already existing mailman subscriber or user a `409` code with a message `user already exists` is returned by the postorius but when the same email but now case sensitive is used a `500` code with message `server error code` is returned.
The latter should also return a `409` instead of `500` as it is misleading.