Python使用纯文本或html发送电子邮件，但不能同时使用两者

import smtplib
from email.mime.text import MIMEText
from email.header import Header

# 邮件内容
msg = MIMEText('这是一封纯文本邮件', 'plain', 'utf-8')
msg['From'] = Header('发件人', 'utf-8')
msg['To'] = Header('收件人', 'utf-8')
msg['Subject'] = Header('邮件主题', 'utf-8')

# 发送邮件
smtp = smtplib.SMTP('smtp.example.com')
smtp.login('username', 'password')
smtp.sendmail('sender@example.com', 'receiver@example.com', msg.as_string())
smtp.quit()

import smtplib
from email.mime.text import MIMEText
from email.header import Header

# 邮件内容
msg = MIMEText('<h1>这是一封HTML邮件</h1>', 'html', 'utf-8')
msg['From'] = Header('发件人', 'utf-8')
msg['To'] = Header('收件人', 'utf-8')
msg['Subject'] = Header('邮件主题', 'utf-8')

# 发送邮件
smtp = smtplib.SMTP('smtp.example.com')
smtp.login('username', 'password')
smtp.sendmail('sender@example.com', 'receiver@example.com', msg.as_string())
smtp.quit()