Bumping the thread a bit to give the latest settings that works (Nov 2024):

    // SMTP configuration
    $mail->isSMTP();                       // Use SMTP
    $mail->Host = 'mailout.one.com';       // Specify your SMTP server
    $mail->SMTPAuth = true;                // Enable SMTP authentication
    $mail->Username = '[email protected]'; // Your One.com email
    $mail->Password = 'password';          // Your One.com email password
    $mail->SMTPSecure = 'tls';             // Enable TLS encryption, 'ssl' is also accepted
    $mail->Port = 587;                     // TCP port to connect to
            
    $mail->CharSet = 'UTF-8';              // Set the character encoding to UTF-8
    $mail->isHTML(true);                   // This ensures the email is sent in HTML format 

Note:

• mailout.one.com - is used for sending mails from your domain.
• send.one.com - is used when you want to send mails with an e-mail client.