用Python SMTP发送我的第一封电子邮件-使用"with“是无效的语法吗？

import smtplib
from email.mime.text import MIMEText

# 邮件内容
msg = MIMEText("这是一封测试邮件", "plain", "utf-8")
msg["Subject"] = "测试邮件"
msg["From"] = "sender@example.com"
msg["To"] = "recipient@example.com"

# SMTP服务器配置
smtp_server = "smtp.example.com"
smtp_port = 25
smtp_username = "your_username"
smtp_password = "your_password"

# 发送邮件
with smtplib.SMTP(smtp_server, smtp_port) as smtp:
    smtp.login(smtp_username, smtp_password)
    smtp.sendmail(msg["From"], msg["To"], msg.as_string())