這是雲端任務裡用的完整代碼，不依賴任何外部庫，只用 Python 標準庫：

import urllib.request, urllib.parse, json, base64, datetime
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText

# OAuth 憑證（直接內嵌，雲端無法用 gcloud）
CLIENT_ID = 'your-client-id.apps.googleusercontent.com'
CLIENT_SECRET = 'your-client-secret'
REFRESH_TOKEN = 'your-refresh-token'

# Step 1: 用 refresh_token 換取 access_token
tok_data = urllib.parse.urlencode({
    'client_id': CLIENT_ID,
    'client_secret': CLIENT_SECRET,
    'refresh_token': REFRESH_TOKEN,
    'grant_type': 'refresh_token'
}).encode()
with urllib.request.urlopen(
    urllib.request.Request('https://oauth2.googleapis.com/token',
                           data=tok_data, method='POST')
) as r:
    access_token = json.loads(r.read())['access_token']

# Step 2: 組裝郵件
msg = MIMEMultipart('alternative')
msg['Subject'] = '測試郵件'
msg['From'] = 'thomastangnz@gmail.com'
msg['To'] = 'thomastangnz@gmail.com'
msg.attach(MIMEText('
Hello!
', 'html', 'utf-8'))
raw = base64.urlsafe_b64encode(msg.as_bytes()).decode()

# Step 3: 用 Gmail API 發送
with urllib.request.urlopen(urllib.request.Request(
    'https://gmail.googleapis.com/gmail/v1/users/me/messages/send',
    data=json.dumps({'raw': raw}).encode(),
    headers={'Authorization': f'Bearer {access_token}',
             'Content-Type': 'application/json'},
    method='POST'
)) as r:
    result = json.loads(r.read())
print('Sent! ID:', result.get('id'))