SMTP email does not work even with the right firewall rules

Question

I am using Mailer Send as my SMTP service and the Fluent SMTP plugin. But it does not matter the plugin with every SMTP plugin I get this error. When I try to send emails, I always get this error:

{
  "code": 423,
  "message": "SMTP Error: Could not connect to SMTP host. Failed to connect to server",
  "errors": [
    "SMTP Error: Could not connect to SMTP host. Failed to connect to server"
  ]
}

I have port 587 open in my firewall and my hosting provider allows email to be sent through this port. What can I do to solve this problem? Thank you for your answers.

Answer 1

Make the following test.

1. Disable all of your SMTP plugins.
2. Temporarily enable WP_DEBUG and WP_DEBUG_LOG in your "wp-config.php" file.
3. Insert the following code in your theme "functions.php" file:

add_action( 'phpmailer_init', function ( $phpmailer ) {
    $phpmailer->isSMTP();
    $phpmailer->Port       = 587;
    $phpmailer->SMTPAuth   = true;
    $phpmailer->SMTPSecure = 'tls';
    $phpmailer->FromName   = get_bloginfo( 'name' );

    // Type your SMTP credentials below.
    $phpmailer->From = ''; // This is the email address that will be used as sender.
    $phpmailer->Host = '';
    $phpmailer->Username = '';
    $phpmailer->Password = '';

    // Temporarily enable while debugging.
    $phpmailer->SMTPDebug = 1;
}, PHP_INT_MAX );

4. Type your credentials in the snippet above and try to send an email. For example, try to reset your password.
5. Did the message reach your inbox?
6. If not, what's inside the file "debug.log" (".../wp-content/debug.log")?