PHP Mail Transport Test - Swiftmailer & Symfony Mailer standalone scripts, instantly usable without frameworks

swiftmailer.php

<?php

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

// composer require swiftmailer/swiftmailer
require_once 'vendor/autoload.php';

// Create the Transport
$transport = (new Swift_SmtpTransport('10.10.10.1', 25));

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

// Create a message
$message = (new Swift_Message('Testmail using Swiftmailer Standalone'))
  ->setFrom(['sender@example.com' => 'John Doe'])
  ->setTo(['recipient@example.com'])
  ->setBody('Here is the message itself');

// Send the message
$result = $mailer->send($message);

symfonymailer.php

<?php

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

// composer require symfony/mailer
require_once 'vendor/autoload.php';

// Create the Transport
$transport = Symfony\Component\Mailer\Transport::fromDsn('smtp://10.10.10.1:25');
//$transport = Symfony\Component\Mailer\Transport::fromDsn('smtp://10.10.10.1:25?verify_peer=0');

// Create the Mailer using your created Transport
$mailer = new Symfony\Component\Mailer\Mailer($transport); 

// Create a message
$email = (new Symfony\Component\Mime\Email())
   ->from('sender@example.com')
   ->to('recipient@example.com')
   ->priority(Email::PRIORITY_HIGHEST)
   ->subject('Testmail using Symfony Mailer Standalone')
   ->text('Here is the message itself')
   ->html('<strong>Here is the message itself</strong>');

// Send the message
$mailer->send($email);

Swiftmailer is EOL since 2021/11. Symfony Mailer is the official successor.

The above scripts are usefull to debug mail transport on servers (most often firewall setups), without initializing a whole framework or mail GUIs.