This error occurs when emailing via the SMTP server without authenticating with it or when the authentication credentials are invalid.
To resolve the error, ensure to configure SMTP authentication. Verify that you have provided the correct username and password provided by the SMTP server. Furthermore, ensure you connect to the correct port using the proper authentication type. For example, if the SMTP requires port 465, use SSL authentication and if the server requires port 587, use TLS authentication.
If nothing works, disable your firewall or anti-virus guard and try sending the email.
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