Maybe just a quick apology? It depends how bad you think it is or to what extent the language might get you reported?
Fess up
On the lines of .... I sent an email addressed to you on x date. I apologise because the tone of the content of the email is quite curt, and may have caused you some distress. It was never my intention to ever cause you any offence. Please accept my apologies and in future I will recheck any communication prior to pressing the 'send' button.
or Deflect blame (to a certain extent)
I wish to apologise for the rude email you may have received from my email account.
I am so sorry if this caused you any distress. I was feeling upset at the time and had composed a message, but thought I had deleted it. I never had any intentions of sending it, and have managed to press the send button by accident. If you need me to apologise personally I will do this as soon as I am able, or whenever there may be a convenient time.
or Ignore it and wait for the fall out