Draft: Add EmailAdjuster

Closes #3419179

src/Plugin/EmailAdjuster/WebformHeadersAdjuster.php

+<?php
+
+namespace Drupal\webform\Plugin\EmailAdjuster;
+
+use Drupal\symfony_mailer\EmailInterface;
+use Drupal\symfony_mailer\Processor\EmailAdjusterBase;
+
+/**
+ * Defines the Webform headers adjuster.
+ *
+ * @EmailAdjuster(
+ *   id = "mailer_webform_sender_header",
+ *   label = @Translation("Webform headers"),
+ *   description = @Translation("Set Webform headers."),
+ *   weight = 100,
+ * )
+ */
+class WebformHeadersAdjuster extends EmailAdjusterBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function postRender(EmailInterface $email) {
+    $from = $email->getFrom();
+    if (count($from) > 0) {
+      $email->setSender($from[0]);
+    }
+  }
+
+}