Issue #3233579 by AdamPS: Upgrade path from SwiftMailer

src/Entity/MailerTransport.php

@@ -118,6 +118,7 @@ class MailerTransport extends ConfigEntityBase implements MailerTransportInterfa
    */
   public function setPluginId($plugin) {
     $this->plugin = $plugin;
+    return $this;
   }
 
   /**
@@ -132,6 +133,7 @@ class MailerTransport extends ConfigEntityBase implements MailerTransportInterfa
    */
   public function setAsDefault() {
     \Drupal::service('config.factory')->getEditable('symfony_mailer.settings')->set('default_transport', $this->id())->save();
+    return $this;
   }
 
   /**

src/MailerTransportInterface.php

@@ -59,6 +59,8 @@ interface MailerTransportInterface extends ConfigEntityInterface {
    *
    * @param string $plugin_id
    *   The transport plugin ID.
+   *
+   * @return $this
    */
   public function setPluginId($plugin_id);
 
@@ -72,6 +74,8 @@ interface MailerTransportInterface extends ConfigEntityInterface {
 
   /**
    * Sets this as the default transport.
+   *
+   * @return $this
    */
   public function setAsDefault();
 

src/Plugin/EmailBuilder/SwiftmailerEmailBuilder.php

+<?php
+
+namespace Drupal\symfony_mailer\Plugin\EmailBuilder;
+
+use Drupal\symfony_mailer\Entity\MailerTransport;
+use Drupal\symfony_mailer\MailerHelperTrait;
+use Drupal\symfony_mailer\Processor\EmailProcessorBase;
+use Drupal\symfony_mailer\Processor\MailerPolicyImportInterface;
+
+/**
+ * Defines the Email Builder plug-in for swiftmailer module.
+ *
+ * Dummy class for config import only.
+ *
+ * @EmailBuilder(
+ *   id = "swiftmailer",
+ *   migrate = @Translation("Swiftmailer transport settings"),
+ * )
+ */
+class SwiftmailerEmailBuilder extends EmailProcessorBase implements MailerPolicyImportInterface {
+
+  use MailerHelperTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function import() {
+    $settings = $this->helper()->config()->get('swiftmailer.transport')->get();
+
+    if ($settings['transport'] == SWIFTMAILER_TRANSPORT_SMTP) {
+      $config = [
+        'user' => $settings['smtp_credentials']['swiftmailer']['username'],
+        'pass' => $settings['smtp_credentials']['swiftmailer']['password'],
+        'host' => $settings['smtp_host'],
+        'port' => $settings['smtp_port'],
+      ];
+
+      $transport = MailerTransport::load('swiftmailer') ?? MailerTransport::create(['id' => 'swiftmailer']);
+      $transport->setPluginId('smtp')
+        ->set('label', 'Imported from swiftmailer')
+        ->set('configuration', $config)
+        ->setAsDefault()
+        ->save();
+    }
+  }
+
+}