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SMTP codes

SMTP 5.7.9 Authentication mechanism too weak

The login method is not allowed; Gmail asks for an app password.

5.7.9
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 3463

What to do

How it appears in a bounce

550 5.7.9 Authentication mechanism too weak

Paste the full bounce into the decoder — it finds every code and recipient and says what to check.

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