Email sending error - 535 Authentication Failed

If you encounter a 535 Authentication Failed error when sending emails, or your email client repeatedly asks for your username and password, this article explains why it happens and provides step-by-step instructions to resolve it.

Issue

When you send an email, your email client may repeatedly prompt you for your email credentials, and you may see the 535 Authentication Failed error. Your email will remain in your Outbox until the issue is resolved. 

This error may appear in the following places: 

• Email clients such as Microsoft Outlook, Mozilla Thunderbird, or Apple Mail when sending emails. 
• Webmail platforms like Roundcube after attempting to send a message. 
• Mail server logs or bounce messages in hosting panels such as cPanel & WHM or Plesk. 

Common error messages 

535 Authentication Failed

535 5.7.8 Authentication credentials invalid

535 Incorrect authentication data

SMTP Error: 535 Authentication Failed

Cause

The 535 Authentication Failed error occurs when the email server cannot validate the credentials used for SMTP authentication. Common causes include: 

• Incorrect username or password 
• Using an incomplete username (for example, user instead of user@(yourdomain.com)) 
• Recently changed email password that was not updated in the email client 
• Incorrect SMTP server settings (wrong server address, port, or encryption type) 
• Temporary server lock after multiple failed login attempts 
• The email account being suspended or restricted 

Solution

To resolve the 535 Authentication Failed error, verify and update the email configuration settings in your email client.
 

1. Confirm the username format 
   Ensure the username is the full email address (e.g., user@(yourdomain.com)).
2. Verify the password
   Re-enter the correct email password in your email client. If unsure, reset the email password. Once done, update your settings in the email client.
    
3. Check SMTP settings
   Confirm the outgoing mail server settings are correct. Typical settings include: 
   • SMTP Server: mail.(yourdomain.com) 
   • Port: 465 (SSL) or 587 (TLS) 
   • Authentication: Enabled 
   • Encryption: SSL or TLS 
   • Username: Full email address 
   • Password: Email password 
   
   Read our Email Setup Guide to find out the exact SMTP settings for your email.
   
    
4. Test using webmail
   Log in to your email through webmail and attempt to send a message. 
   • If it works, the issue is likely related to the email client's configuration. 
   • If it fails, the email account or server may require assistance from the hosting provider.