diff -u b/src/Plugin/Mail/SwiftMailer.php b/src/Plugin/Mail/SwiftMailer.php
--- b/src/Plugin/Mail/SwiftMailer.php
+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -588,10 +588,11 @@
     // separated by the mail line endings. Keep Markup objects and escape others
     // and then treat the result as safe markup.
     $line_endings = Settings::get('mail_line_endings', PHP_EOL);
-    $message['body'] = Markup::create(implode($line_endings, array_map(function ($body) {
-      // If the body contains no html tags we can assume newlines will need be
-      // converted to <br>.
-      if (strlen(strip_tags($body)) === strlen($body)) {
+    $applicable_format = $this->getApplicableFormat($message);
+    $message['body'] = Markup::create(implode($line_endings, array_map(function ($body) use ($applicable_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)) {
         // The default fallback format is 'plain_text', which escapes markup,
         // converts new lines to <br> and converts URLs to links.
         $build = [