First of all, your question is not a future request. I've moved it to the appropriate board, but urge you to be more attentive next time.
And I don't understand your question. What do you mean by 'on' and 'off'? If at least one operator is available, a visitor can chat. If there are no available operators, a visitor can leave a message.
To get email notifications from visitors when there are no available operators one should just set the valid email address in the settings of Mibew Messenger.