SendGrid with Flask: Flask-Mail SMTP config

app.config.update(
  MAIL_SERVER='smtp.sendgrid.net',
  MAIL_PORT=587,
  MAIL_USERNAME='apikey',  # literal string, not your key
  MAIL_USE_TLS=True,
  MAIL_DEFAULT_SENDER='[YOUR_VERIFIED_SENDER]'  # verified sender
)

Set MAIL_PASSWORD from the SENDGRID_API_KEY environment variable (for example via os.environ when building config). Do not hardcode it.

Then:

from flask_mail import Mail, Message
mail = Mail(app)
msg = Message('subject here', recipients=['[RECIPIENT_EMAIL]'])
msg.body = 'plain body'
mail.send(msg)

Checklist:
1. MAIL_USERNAME is exactly 'apikey'.
2. MAIL_USE_TLS True with port 587. Port 465 wants MAIL_USE_SSL instead.
3. MAIL_DEFAULT_SENDER must be a verified sender.
4. In tests, suppress sending (MAIL_SUPPRESS_SEND) or point at sandbox mode so CI stays quiet.