The ‘Error 535’ issue often occurs if the credentials you entered when setting up your SMTP plugin are invalid.
Most email servers require users to authenticate their connection using an API key. If there is a mismatch between the API key on your SMTP plugin and the one on your SMTP server, you’ll likely encounter this error.
Resolving the Error – Solution
To resolve this error, we suggest verifying that you’ve copied the correct authentication credentials from your SMTP server.
If you’re using an SMTP plugin such as Easy WP SMTP, you may need to run through the setup process for your chosen email server.