complete a written response to this peer

After realizing the mistake, I would immediately contact the attorney I accidentally emailed. They have a code of ethics as well, and might not even open the email if they're aware of it ahead of time. I'd then notify the client of my mistake and let them know It's being handled and won't happen again. Finally, I would email the attorney it was originally intended for. Mistakes happen. I would learn to be a little more cautious before hitting send and not let it happen again.