异常:发送电子邮件失败:没有收件人，不知道出了什么问题

import smtplib
from email.mime.text import MIMEText
import re

def is_valid_email(email):
    email_regex = r'^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$'
    return re.match(email_regex, email) is not None

def send_email(sender, recipient, subject, body):
    if not is_valid_email(recipient):
        raise ValueError("Invalid recipient email address")

    msg = MIMEText(body)
    msg['Subject'] = subject
    msg['From'] = sender
    msg['To'] = recipient

    try:
        smtp_server = smtplib.SMTP('smtp.example.com', 587)
        smtp_server.starttls()
        smtp_server.login(sender, 'password')
        smtp_server.sendmail(sender, recipient, msg.as_string())
        smtp_server.quit()
        print("Email sent successfully")
    except Exception as e:
        print(f"Failed to send email: {e}")

# 示例调用
sender = 'your_email@example.com'
recipient = 'recipient_email@example.com'
subject = 'Test Email'
body = 'This is a test email.'

send_email(sender, recipient, subject, body)