While I have no specific information about Mount Sinai, I doubt that any school would do anything so tacky as sending a rejection by email!! All of my rejections have been through snail mail (well, except the one I found out about on the phone because I called the JHU admissions office so much about trying to coordinate interviews). Interview invites have come by email, snail mail and three times by phone (in which cases they also later sent the email or paper letter).