msg['to'] = TO_ADDRESS
msg['from'] = FROM_ADDRESS
msg['subject'] = 'testing reply-to header'
msg.add_header('reply-to', REPLY_TO_ADDRESS)
server = smtplib.SMTP(MAIL_SERVER)
server.sendmail(msg['from'], [msg['to']], msg.as_string())
Setting different reply-to message in Python email/smtplib