535
SMTPERRORNotableAuthenticationHIGH confidence
Authentication credentials invalid
What this means
The 535 code is a direct and permanent failure indicating the username or password provided during authentication is incorrect. The server has rejected the credentials.
Why it happens
- 1The client provided the wrong username or password.
- 2The client used an incorrect encoding for the credentials (should be base64).
- 3The user's account is locked or disabled on the server.
How to reproduce
A user provides an incorrect password when their email client tries to send a message.
trigger — this will error
trigger — this will error
AUTH LOGIN 334 VXNlcm5hbWU6 dXNlckBleGFtcGxlLmNvbQ== 334 UGFzc3dvcmQ6 d3JvbmdwYXNzd29yZA== 535 5.7.8 Authentication credentials invalid
expected output
535 Authentication credentials invalid
Fix
Verify username and password
WHEN When configuring an email client or application
Verify username and password
// Ensure credentials are correct before connecting
const config = {
user: "correct-user@example.com",
pass: "correct-password-123"
};Why this works
Correct credentials are required for authentication. This is the most common fix.
What not to do
✕ Retry repeatedly with the same credentials
This can lead to the client's IP address being blocked by security systems like fail2ban.
Sources
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