The error "SMTP error: could not authenticate" may occur for the following reasons:

1. If the configured SMTP server is Gmail, the error may result from Gmail's default settings blocking less secure applications.
2. Incorrect credentials for the configured SMTP Microsoft Exchange Server account.
   

To resolve the authentication failure issue, follow the steps below:

1. Enable Less Secure Apps for Gmail: Navigate to "My Accounts" in your email settings.

1. Select the "Sign-in & Security" option.
2. Enable "Allow less secure apps" by setting it to ON.
   

2. Provide Correct Credentials: Ensure that the correct credentials are provided for your SMTP Exchange server.

3. Reconfigure Email Settings: After completing the above steps, reconfigure the email settings in your BDRShield web console.

4. Send Test Email: Send a test email and verify that it is successful.