Alternative to swiftmailer in laravel

I have an issue with using swiftmailer with the client's SMTP microsoft exchange server.

To troubleshoot, I am thinking of changing out the swiftmailer with something else.

My entire setup is on an internal network. So no mailgun or sendgrid at all.

How do I use an alternative library for swiftmailer in Laravel in order to troubleshoot?

Also this is the log I get when I tried to use swiftmailer on Laravel

<< 250-xxx.internal.abc.com Hello [xxx.xx.xxx.xx]
250-SIZE 36700160
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-AUTH
250-8BITMIME
250-BINARYMIME
250-CHUNKING
250-XEXCH50
250 XSHADOW

>> STARTTLS

<< 500 5.3.3 Unrecognized command

!! Expected response code 220 but got code "500", with message "500 5.3.3 Unrecognized command

What I tried part (I)

Somebody asked me to try MAIL_ENCRYPTION to null and dd(config('mail.encryption')), so the below is the answer:

Code:

    try { 
        Mail::send('emails.test', [], function ($m) {
        $m->from('xxx@client.com', 'Your Application')->to('me@me.com');
        dd(config('mail.encryption'));
        var_dump('executed');
    });
     } catch (Exception $e) {
        var_dump($e->getMessage()); 
        var_dump($e->getTraceAsString());
    }  

Result: null was printed out The string executed was not.

What I tried part (II):

explicitly use swiftmailer transport with encryption set to null

  try {
    // Create the Transport
    $transport = \Swift_SmtpTransport::newInstance();

    $transport->setUsername('xxx@client.com')
    ->setPort(25)
    ->setHost('smtp.internal.client.com')
    ->setEncryption(null);

    // Create the Mailer using your created Transport
    $mailer = \Swift_Mailer::newInstance($transport);

    $logger = new \Swift_Plugins_Loggers_ArrayLogger();
    $mailer->registerPlugin(new \Swift_Plugins_LoggerPlugin($logger));


    $message = new \Swift_Message('Wonderful Subject');
    $message->setFrom(['xxx@client.com' => 'John Doe']);
    $message->setTo(['me@me.com'])->setBody('Here is the message itself');

    // // Send the message
    $numSent = $mailer->send($message);
    } catch(\Swift_TransportException $e) {
        var_dump('test');
        var_dump($e->getMessage()); 
        var_dump($e->getTraceAsString());
    }

Result:

www-data@server:~/html/laravel-app$ php artisan email:send
string(4) "test"
string(127) "Failed to authenticate on SMTP server with username "xxx@client.com" using 0 possible authenticators"
string(2503) "#0 /var/www/html/a3qp/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php(332): Swift_Transport_Esmtp_AuthHandler->afterEhlo(Object(Swift_SmtpTransport))

What I tried Part (III):

Same as part II but set encryption to 'tls' explicitly. I get the same error message as the original one at the very top.

What I tried Part (IV):

Someone asked me to take out the dd line in Part I This is what I got

string(8) "executed"
[Swift_TransportException]
Failed to authenticate on SMTP server with username "xxx@client.com" using 0 possible authenticators

Update

1. I am using SMTP to a MS Exchange server that is in my client's infrastructure. I have no access to that server's SMTP log, so it's hard to troubleshoot. The code is totally intranet. There is no access to outside internet service and I am not supposed to use mailgun or any other smtp cloud services to send client's transactional emails meant for internal consumption.
2. I have another code using a legacy cakephp 2 system that can work with the same SMTP mail server with no issues.
3. I have tested my laravel SMTP settings on a google mail. And it works fine.
4. My next step for troubleshooting is to see if it's swiftmailer or the rest of laravel where the issue lies. Hence this post.
5. Yes, I am trying to get hold of the IT team responsible for the Exchange server, but they are in another country. I am still trying though.
6. I know I am repeating point 2, but the comments suggest that this point is not clear enough. I am using the exact same SMTP settings on a legacy cakephp 2 system to send emails and there is no issue. Because I cannot get hold of the IT team responsible for the exchange server and plus the SMTP settings do work for another cakephp 2 system, I feel I should try to troubleshoot it on the laravel system to find out for sure.
7. I have tried PHP mail and sendmail by changing the .env file. I did not trigger any error messages but i also appear not to receive any emails.
8. Please see What I tried Part I, Part II, Part III, Part IV for details on code used to test the SMTP