Add the ability to configure whether to keep successful mail statuses or not

Right now when using the Database Mail Listener the statuses for all mails are kept (whether they've been sent successfully or not). This is good for tracability but admins may not need/want tracability and may only need to keep failing mail statuses.

It will also be good to turn off keep track of successfully sent mail by default to reduce to size of the database table by default and not have to worry about maintenance by default.