diff -u b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php
--- b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php
+++ b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php
@@ -76,8 +76,23 @@
         unset($message['headers']['Return-Path']);
       }
     }
+
     $headers = new Headers();
+    // Format the data for Symfony header handlers.
+    // @see \Symfony\Component\Mime\Header\Headers::HEADER_CLASS_MAP
     foreach ($message['headers'] as $name => $value) {
+      switch (strtolower($name)) {
+        case 'from':
+        case 'to':
+        case 'reply-to':
+        case 'cc':
+        case 'bcc':
+          $value = is_array($value) ? $value : [$value];
+          break;
+
+        default:
+          break;
+      }
       $headers->addHeader($name, $value);
     }
     $line_endings = Settings::get('mail_line_endings', PHP_EOL);