使用smptlib发送的电子邮件缺少主题行

import smtplib
from email.mime.text import MIMEText

# 邮件内容
msg = MIMEText('这是一封测试邮件。')

# 添加主题行
msg['Subject'] = '测试邮件主题'

# 发件人和收件人
msg['From'] = 'sender@example.com'
msg['To'] = 'recipient@example.com'

# 发送邮件
smtp_server = 'smtp.example.com'
smtp_port = 25
smtp_username = 'username'
smtp_password = 'password'

try:
    server = smtplib.SMTP(smtp_server, smtp_port)
    server.login(smtp_username, smtp_password)
    server.sendmail(msg['From'], msg['To'], msg.as_string())
    server.quit()
    print('邮件发送成功')
except Exception as e:
    print('邮件发送失败:', str(e))