解决方法：
发送的消息体加入键值对：

"From":"xxxx.com"
"To":"xxxx.com"

from smtplib import SMTP
from email.header import Header
from email.mime.text import MIMEText
from email.mime.image import MIMEImage
from email.mime.multipart import MIMEMultipart

import urllib

def main():
    message = MIMEMultipart()

    text_content = MIMEText('附件中带有数据，请查收', 'plain', 'utf-8')
    message['Subject'] = Header('本月数据', 'utf-8')
    message['From'] = 'xxxx@126.com'
    message['To'] = 'xxxx@qq.com'
    message.attach(text_content)

    with open('email_.txt', 'rb') as f:
        txt = MIMEText(f.read(), 'base64', 'utf-8')
        txt['Content-Type'] = 'text/plain'
        txt['Content-Disposition'] = 'attachment; filename=email_.txt'
        message.attach(txt)

    with open('email_.xlsx', 'rb') as xf:
        xls = MIMEText(xf.read(), 'base64', 'utf-8')
        xls['Content-Type'] = 'application/vnd.ms-excel'
        xls['Content-Disposition'] = 'attachment; filename=email_.xlsx'
        message.attach(xls)

    smtper = SMTP('smtp.126.com')
    smtper.starttls()
    sender = 'xxxx@126.com'
    receiver = ['xxxx@qq.com']
    # xxxx: 相应邮箱的客户端授权码
    smtper.login(sender, 'xxxx')
    smtper.sendmail(sender, receiver, message.as_string())
    smtper.quit()
    print('发送成功！')


if __name__ == '__main__':
    main()