SMTP codes
SMTP 534 Authentication mechanism too weak
The chosen login method is too weak, or the provider wants an app password / OAuth. Gmail answers 534 5.7.9 when an app password is needed.
- Class
- 5xx · Permanent failure
- Retried?
- No, the message bounces
- Defined in
- RFC 4954
What to do
- Create an app password (Gmail, Yandex, Mail.ru) or use OAuth.
- Use TLS so the server allows PLAIN/LOGIN.
How it appears in a bounce
534 Authentication mechanism too weak
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.