In cPanel Exim servers, especially ones enabled with SMTP authentication, “535 Incorrect authentication data” is a common error message seen in mail logs.
The vast majority of these error instances represent blocked connections from botnets trying to relay spam. This is a security feature to protect the mail server from spam, and cannot be disabled.
- “Server says: 535 – Incorrect authentication data” error is usually related to the SMTP User Authentication details you have configured (or not configured) in your email settings.
- This error is often reported by mail users who maintain multiple mail devices, and forget to update login details in one of their devices.
- However, in very rare cases 535 Incorrect Authentication Data’ results when mailbox permissions are changed due to migrations, or an account/server update etc, in cases like this running "Repair Mailbox permissions" which should resolve the issue if that's the case.