Wrong case for headers after using MailManagerInterface::mail() issue fixed

modules/mailer_override/src/LegacyMailerHelper.php

@@ -80,16 +80,21 @@ class LegacyMailerHelper implements LegacyMailerHelperInterface {
     }
 
     foreach ($email->getHeaders()->all() as $name => $header) {
-      $lc_name = strtolower($name);
-      if (isset(self::SKIP_HEADERS[$lc_name])) {
+      // Symfony stores headers in lower-case.
+      if (isset(self::SKIP_HEADERS[$name])) {
         continue;
       }
 
-      // Copy the header.
-      $message['headers'][$name] = $header->getBodyAsString();
-      if (!empty(self::ADDRESS_HEADERS[$lc_name])) {
-        // Also copy directly to $message.
-        $message[$lc_name] = $message['headers'][$name];
+      // Drupal message array stores headers in camel-case, except 'Reply-to'
+      // is non-standard.
+      $legacy_name = ($name == 'reply-to') ? 'Reply-to' : ucwords($name, '-');
+
+      // Copy to 'headers'.
+      $message['headers'][$legacy_name] = $header->getBodyAsString();
+
+      if (!empty(self::ADDRESS_HEADERS[$name])) {
+        // Also copy directly to $message, as lower-case.
+        $message[$name] = $message['headers'][$legacy_name];
       }
     }