在python中发送带有MIME的pdf附件的电子邮件失败

import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.base import MIMEBase
from email import encoders

# 邮件服务器配置
smtp_host = 'smtp.example.com'
smtp_port = 587
smtp_username = 'your_username'
smtp_password = 'your_password'

# 发件人和收件人
sender = 'sender@example.com'
receiver = 'receiver@example.com'

# 创建邮件对象
msg = MIMEMultipart()
msg['From'] = sender
msg['To'] = receiver
msg['Subject'] = '带有PDF附件的邮件'

# 添加正文
body = '这是一封带有PDF附件的邮件。'
msg.attach(MIMEText(body, 'plain'))

# 添加附件
attachment_path = 'path/to/attachment.pdf'
attachment = open(attachment_path, 'rb')
part = MIMEBase('application', 'octet-stream')
part.set_payload((attachment).read())
encoders.encode_base64(part)
part.add_header('Content-Disposition', "attachment; filename=attachment.pdf")
msg.attach(part)

# 发送邮件
try:
    server = smtplib.SMTP(smtp_host, smtp_port)
    server.starttls()
    server.login(smtp_username, smtp_password)
    server.sendmail(sender, receiver, 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))