I am learning how to use Symfony Mailer and am a bit lost on the encrypting message section.

This is my simple code below which works in sending the email.

<?php

require 'vendor/autoload.php';

const FROM = 'me@example.com';

const TO = 'example@email.com';

const SUBJECT = 'My Subject';

const MESSAGE = 'Hello World';

const DSN = 'smtp://localhost:1025';

$transport = \Symfony\Component\Mailer\Transport::fromDsn(DSN);

$mailer = new \Symfony\Component\Mailer\Mailer($transport);

$email = (new \Symfony\Component\Mime\Email())
    ->from(FROM)
    ->to(TO)
    ->subject(SUBJECT)
    ->text(MESSAGE);

$encrypter = new \Symfony\Component\Mime\Crypto\SMimeEncrypter('my-certificate.crt');
$encryptedEmail = $encrypter->encrypt($email);

try {
    $mailer->send($encryptedEmail);
} catch (\Symfony\Component\Mailer\Exception\TransportExceptionInterface $error) {
    echo 'Unable to send email' . PHP_EOL;
}

And this is how I generated the certificate and key...

openssl genrsa -aes256 -out my-certificate.key 4096
openssl req -new -x509 -days 29220 -key my-certificate.key -out my-certificate.crt

I am able to receive the email using SMTP tools like Mailpit.

My two questions are...

1. My emails are encrypted using the certificate, but shouldn't it be done using PGP?
2. How do I decrypt the email with SMTP testing tool or any online or CLI tool? I tried to decrypt the email and could not decrypt it even though I have all of the keys.