How to fix SMTP Error 535 5.7.0

SMTP Error 535 5.7.0 is a permanent delivery failure caused by authentication issues, specifically from the sender side. It means the server refuses to accept credentials or authentication used to send the email. To resolve this issue, senders can follow any of these solutions:

1. Confirm credentials
2. Use app-specific passwords instead of account passwords
3. Verify server and port settings
4. Match updated security policies of provider
5. Try again after throttling periods

As part of the general SMTP error 535, Error 535 5.7.0 means the server refuses to deliver your email due to a specific error with the authentication attempt. The mail server did not accept the credentials or authentication behavior provided by the sending client.

Here is what happens behind the scenes:

• In SMTP authentication, the sender must prove their identity to the email server (often using AUTH LOGIN or a similar mechanism). 
• If that authentication fails, the server will refuse to move forward and return a 535 error. 

You might see messages like:

• 535 5.7.0 – Too many authentication failures
• 535 5.7.0 – Authentication failed

How to fix “SMTP Error 535 5.7.0?”

Since Error 535 5.7.0 is tied to authentication, the fixes revolve around correct credentials and secure authentication practices:

1. Confirm credentials. Double-check that the username and password are correct and up to date. If you’ve changed the password recently, update it in your mail client or sending application.
2. Use app-specific passwords. For providers with strict security (Gmail, Yahoo, AOL), generate an app-specific password instead of using the account password. This overrides blocks on basic authentication. 
3. Verify server and port settings. Ensure you’re connecting to the correct SMTP server and port: Port 587 with STARTTLS for modern clients and Port 465 with SSL/TLS where required
4. Support provider security rules. If your provider has updated security policies (e.g., OAuth enforcement or blocked basic auth), configure your client to match those requirements or use a supported app integration.
5. Retry after throttling periods. In cases where the server blocks authentication attempts due to too many failures, waiting a short period before trying again may resolve the issue.