SMTP Error 535: Authentication Failed (How to Fix It)

Open the bounce message or email app error details. 

Ensure it specifically references: 

535 Authentication failed 
535 Incorrect authentication data 
535 5.7.8 Username and Password not accepted 

If it shows a different SMTP code, refer to our SMTP Error Codes guide. 

Your username must be your full email address. 

• Correct: [email protected] 
• Incorrect: name 

Update this in: 

• Email App → Account Settings → Outgoing Server (SMTP) 

• Re-enter the password carefully 
• Avoid copy/paste errors 
• Check Caps Lock 

If unsure, reset the email password in your hosting control panel and update it on all devices. 

In your outgoing mail settings: 

• Authentication must be enabled 
• Choose “Use same credentials as incoming mail server” 

If authentication is disabled, the server will reject the connection. 

Use secure SMTP settings: 

If encryption is disabled, authentication may fail. 

After saving changes: 

• Send a test email 
• Close the app completely 
• Reopen it 

What It Means: 
The username or password used for SMTP login is incorrect. 

How to Fix It: 

• Re-enter your password carefully 
• Ensure you are using your full email address as the username 
• Reset the mailbox password in your hosting control panel if unsure 
• Update the password on all connected devices 

Why It Happens: 
Incorrect password, outdated saved credentials, or typing errors. 

What It Means: 
The server rejected the login attempt due to invalid credentials or authentication settings. 

How to Fix It: 

• Confirm SMTP authentication is enabled 
• Verify port 465 (SSL) or 587 (TLS) is being used 
• Check that encryption is enabled 
• Reset the mailbox password and retry 

Why It Happens: 
Authentication is disabled in the email app, the wrong port, or the incorrect encryption settings. 

What It Means: 
The login information sent to the server is malformed or incomplete. 

How to Fix It: 

• Ensure username is the full email address 
• Remove extra spaces before or after the username 
• Re-save the account settings 
• Restart the email application 

Why It Happens: 
Incorrect username format or corrupted account configuration. 

What It Means: 
The mailbox has been temporarily locked after multiple failed login attempts. 

How to Fix It: 

• Wait 15–30 minutes before retrying 
• Reset the mailbox password 
• Update credentials on all devices (phone, laptop, tablet) 
• Restart your router if an IP-based lockout is suspected 

Why It Happens: 
Repeated incorrect password attempts from one or multiple devices. 

What It Means: 
An application (Outlook, Thunderbird, phone mail app, website plugin) is using outdated or incorrect SMTP credentials. 

How to Fix It: 

• Update stored credentials in the application 
• Remove and re-add the email account 
• Confirm the correct outgoing mail server hostname 
• Test sending after updating 

Why It Happens: 
The password was changed in the hosting panel, but not updated in the email client. 

SMTP 535 errors are logged during the authentication phase. 

grep "535" /var/log/exim_mainlog 
tail -f /var/log/exim_mainlog 

authentication failed 
535 Incorrect authentication data 
535 5.7.8 Username and Password not accepted 

grep "sasl" /var/log/maillog 
grep "authentication failed" /var/log/maillog 
tail -f /var/log/maillog 

Goal: 
Confirm whether the failure is due to invalid credentials, disabled authentication, or SASL misconfiguration. 

SMTP 535 often occurs when SASL authentication is disabled or misconfigured. 

cat /etc/postfix/main.cf | grep smtpd_sasl_auth_enable 

systemctl restart postfix 

nano /etc/exim.conf 

systemctl restart exim 

Use Telnet or OpenSSL to test authentication. 

openssl s_client -starttls smtp -connect mail.example.com:587 

535 Authentication failed 

swaks --to [email protected] \ 
--from [email protected] \ 
--server mail.yourdomain.com \ 
--auth LOGIN \ 
--auth-user [email protected] \ 
--auth-password 'password' 

If using virtual mail users, confirm the mailbox exists: 

For cPanel: 
Check via WHM → Email Accounts 

For Postfix + Dovecot Setups: 

cat /etc/dovecot/users 

If password hashes are used, confirm they match the expected authentication method. 

Reset the mailbox password and retest if needed. 

Multiple failed login attempts can trigger temporary locks. 

Check logs for: 

• Too many login attempts 
• Authentication temporarily blocked 

If present: 

• Reset password 
• Wait 15 to 30 minutes 
• Restart mail services if necessary 

Authentication failures can occur if SSL/TLS is mismatched. 

Verify: 

• Port 465 → SSL 
• Port 587 → STARTTLS 
• Encryption is enabled in the mail client 

Test listening ports: 

netstat -tlnp | grep :465 
netstat -tlnp | grep :587 

If ports are closed, check firewall rules. 

Security systems (CSF, Fail2Ban, ModSecurity) may block repeated attempts. 

For CSF: 

csf -g <client-ip> 

Unblock if necessary: 

csf -dr <client-ip> 

Review Fail2Ban: 

fail2ban-client status 

Most commonly: 

• Password change 
• New device setup 
• Security update enforcing SSL

Yes, excessive failed attempts can trigger temporary restrictions.