We need to add an email sending method to osCommerce. In addition to the defaulty PHP send email method, we need to be able to use a separate/remote SMTP server to send emails. It should be also be configured so that: (1). Any failed delivery message will be appended to order status history; (2) Customer's reply to that email will send to the store's main email address instead of the email on the remote SMTP server.
Another minor change is also required: when the admin append a note to the order status history (via Admin Panel), if the admin did not choose the "notify" customer option, then we don't the customer to see the note either when he/she logged in to check order status/history.