I closed the browser by accident. Can i resume the sending process?

If you were sending the newsletter Immediately and you have accidentely closed the browser, then you can go to RSMail! >> Sessions and check the Session that you have started. If it was not finished, you can resume it in order to send the message to your subscribers.

If the messages are sent with a Scheduled Session then the browser does not affect the session, as the messages are sent with the help of a Cron task.