The only way you can show you did not send the email on purpose is by acting differently from this woman who is attacking you on purpose. At the moment, from outside, it looks confusing and as if you could be trying to get-away-with an aggressive act on this woman (sending an ugly email and saying your drugged evil twin did it, not you). If you act horrified at having sent the email and apologetic and "plead" with the school to give you the lighter reprimand for having sent the email with a letter from your doctor saying the prescribed Ambien lowered your inhibitions (not like you were out drinking all night and sent the email under the effects of alcohol), it will have to be over; the other woman won't have any way to add more to the fire because the "official" pronouncement will have been made.