Drupal Symfony Mailer - Custom Header Email Adjuster

CustomHeaderEmailAdjuster.php

<?php

namespace Drupal\MY_MODULE\Plugin\EmailAdjuster;

use Drupal\Core\Form\FormStateInterface;
use Drupal\symfony_mailer\EmailInterface;
use Drupal\symfony_mailer\Processor\EmailAdjusterBase;

/**
 * Defines the Custom header Email Adjuster.
 *
 * @EmailAdjuster(
 *   id = "custom_header",
 *   label = @Translation("Custom Header"),
 *   description = @Translation("Sets the email custom header."),
 * )
 */
class CustomHeaderEmailAdjuster extends EmailAdjusterBase {

  /**
   * {@inheritdoc}
   */
  public function build(EmailInterface $email) {
    $header_name = $this->configuration['name'];
    $header_value = $this->configuration['value'];
    // addHeader or addTextHeader ?
    $email->getHeaders()->addTextHeader($header_name, $header_value);
  }

  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form['name'] = [
      '#title' => $this->t('Custom header name'),
      '#type' => 'textfield',
      '#default_value' => $this->configuration['name'] ?? NULL,
      '#required' => TRUE,
    ];
    $form['value'] = [
      '#title' => $this->t('Custom header value'),
      '#type' => 'textfield',
      '#default_value' => $this->configuration['value'] ?? NULL,
      '#required' => TRUE,
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function getSummary() {
    return $this->configuration['name'] . ': ' . $this->configuration['value'];
  }

}