from email.mime.text import MIMEText
msg = MIMEMultipart('alternative')
msg['Subject'] = '\u30c6\u30b9\u30c8\u30e1\u30fc\u30eb'
part1 = MIMEText('\u3053\u3093\u306b\u3061\u306f\u3001\u4e16\u754c\uff01\n',
    'plain', 'utf-8')
msg.attach(part1)
print(msg.as_string().encode('ascii'))
Python – Как отправить электронную почту utf-8?