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
@@ -21,4 +21,11 @@
 
   /**
+   * A list of headers that can contain multiple email addresses.
+   *
+   * @see \Symfony\Component\Mime\Header\Headers::HEADER_CLASS_MAP
+   */
+  private const MAILBOX_LIST_HEADERS = ['from', 'to', 'reply-to', 'cc', 'bcc'];
+
+  /**
    * The configuration factory.
    *
@@ -78,20 +85,9 @@
     }
 
     $headers = new Headers();
-    // Format the data for Symfony header handlers.
-    // @see \Symfony\Component\Mime\Header\Headers::HEADER_CLASS_MAP
     foreach ($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;
+      if (in_array(strtolower($name), self::MAILBOX_LIST_HEADERS, TRUE)) {
+        $value = explode(',', $value);
       }
       $headers->addHeader($name, $value);
     }