diff --git a/config/install/swiftmailer.message.yml b/config/install/swiftmailer.message.yml
index fa86216..20a47be 100644
--- a/config/install/swiftmailer.message.yml
+++ b/config/install/swiftmailer.message.yml
@@ -1,4 +1,5 @@
 format: text/plain
+filter_format: plain_text
 respect_format: true
 convert_mode: false
 character_set: UTF-8
diff --git a/config/schema/swiftmailer.schema.yml b/config/schema/swiftmailer.schema.yml
index bbfa947..5235b9f 100644
--- a/config/schema/swiftmailer.schema.yml
+++ b/config/schema/swiftmailer.schema.yml
@@ -5,6 +5,9 @@ swiftmailer.message:
     format:
       type: string
       label: 'Message format'
+    filter_format:
+      type: string
+      label: 'Filter format'
     respect_format:
       type: boolean
       label: 'Respect format'
diff --git a/src/Plugin/Mail/SwiftMailer.php b/src/Plugin/Mail/SwiftMailer.php
index eeda5c1..727a667 100644
--- a/src/Plugin/Mail/SwiftMailer.php
+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -589,7 +589,8 @@ public function massageMessageBody(array $message) {
     // and then treat the result as safe markup.
     $line_endings = Settings::get('mail_line_endings', PHP_EOL);
     $applicable_format = $this->getApplicableFormat($message);
-    $message['body'] = Markup::create(implode($line_endings, array_map(function ($body) use ($applicable_format) {
+    $filter_format = $this->config['message']['filter_format'];
+    $message['body'] = Markup::create(implode($line_endings, array_map(function ($body) use ($applicable_format, $filter_format) {
       // If the body contains no html tags but the applicable format is HTML,
       // we can assume newlines will need be converted to <br>.
       if ($applicable_format == SWIFTMAILER_FORMAT_HTML && strlen(strip_tags($body)) === strlen($body)) {
@@ -598,6 +599,7 @@ public function massageMessageBody(array $message) {
         $build = [
           '#type' => 'processed_text',
           '#text' => $body,
+          '#format' => $filter_format,
         ];
         $body = $this->renderer->renderPlain($build);
       }
diff --git a/swiftmailer.install b/swiftmailer.install
index 815138e..1a9b6ab 100644
--- a/swiftmailer.install
+++ b/swiftmailer.install
@@ -32,3 +32,12 @@ function swiftmailer_install() {
     'sender' => 'swiftmailer',
   ])->save();
 }
+
+/**
+ * Set default filter_format for HTML emails with no HTML markup.
+ */
+function swiftmailer_update_8400() {
+ Drupal::configFactory()->getEditable('swiftmailer.message')
+   ->set('filter_format', 'plain_text')
+   ->save();
+}