如何用Python实现毫秒级定时发送Gmail并解决认证失败问题

问题背景 #

需要毫秒级精度定时发送Gmail用于院校申请（接收时间影响候选人排序），Gmail自带定时发送存在延迟，用Python脚本实现时遇到smtplib.SMTPSenderRefused认证错误——原因是Google已禁用非授权应用访问，且关闭了“不太安全的应用”设置。

解决方法 #

方法一：使用应用专用密码（需开启两步验证） #

这是个人账号最简便的解决方案：

• 先为你的Google账号开启两步验证（2FA）
• 登录Google账号安全设置，找到「应用专用密码」选项，选择「邮件」应用与对应设备，生成一个16位的专用密码
• 修改代码，在初始化SMTP服务器后添加登录步骤，用该专用密码替代原账号密码：

import smtplib

gmail_user = "your-email@gmail.com"
gmail_app_password = "your-16-digit-app-password"  # 替换为生成的应用专用密码

def send_mail():
    try:
        server_ssl = smtplib.SMTP_SSL('smtp.gmail.com', 465)
        server_ssl.ehlo()
        # 添加认证登录步骤
        server_ssl.login(gmail_user, gmail_app_password)
        print('Server initialized and authenticated')
    
        sent_from = gmail_user
        to = ['recipient@example.com']
        subject = 'my subject'
        body = 'my body'

        email_text = """\
From: %s
To: %s
Subject: %s

%s
""" % (sent_from, ", ".join(to), subject, body)
        
        server_ssl.sendmail(sent_from, to, email_text)
        server_ssl.close()
        print('Email sent successfully')
    except Exception as inst:
        print('Something went wrong')
        print(type(inst))
        print(inst.args)
        print(inst)

方法二：使用OAuth2认证（开发者规范方案） #

如果不想使用应用专用密码，或需要更安全的认证方式，可采用OAuth2：

1. 前往Google Cloud Console创建新项目，启用Gmail API
2. 创建OAuth客户端ID，选择「桌面应用」类型，下载credentials.json到项目目录
3. 安装依赖库：pip install google-api-python-client oauth2client
4. 使用以下代码发送邮件：

import base64
from email.mime.text import MIMEText
from googleapiclient.discovery import build
from googleapiclient.errors import HttpError
from oauth2client.file import Storage
from oauth2client.client import OAuth2WebServerFlow
from oauth2client.tools import run_flow

# OAuth2配置（从credentials.json提取对应字段）
CLIENT_ID = 'your-client-id'
CLIENT_SECRET = 'your-client-secret'
SCOPES = ['https://www.googleapis.com/auth/gmail.send']
REDIRECT_URI = 'urn:ietf:wg:oauth:2.0:oob'

def get_credentials():
    storage = Storage('gmail-credentials.json')
    credentials = storage.get()
    if not credentials or credentials.invalid:
        flow = OAuth2WebServerFlow(CLIENT_ID, CLIENT_SECRET, SCOPES, REDIRECT_URI)
        credentials = run_flow(flow, storage)
    return credentials

def send_mail():
    try:
        credentials = get_credentials()
        service = build('gmail', 'v1', credentials=credentials)
        
        sent_from = 'your-email@gmail.com'
        to = 'recipient@example.com'
        subject = 'my subject'
        body = 'my body'
        
        message = MIMEText(body)
        message['to'] = to
        message['from'] = sent_from
        message['subject'] = subject
        
        raw = base64.urlsafe_b64encode(message.as_bytes()).decode()
        message_body = {'raw': raw}
        
        service.users().messages().send(userId='me', body=message_body).execute()
        print('Email sent successfully')
    except HttpError as error:
        print(f'An error occurred: {error}')

首次运行时会弹出浏览器要求授权应用访问Gmail，授权后生成gmail-credentials.json，后续运行无需重复授权。

内容的提问来源于stack exchange，提问作者crag.hack