diff --git a/app/models/mailer.rb b/app/models/mailer.rb
index 8be6f83..f59dadc 100644
--- a/app/models/mailer.rb
+++ b/app/models/mailer.rb
@@ -370,7 +370,7 @@ class Mailer < ActionMailer::Base
   end
 
   def mail(headers={}, &block)
-    headers.merge! 'X-Mailer' => 'Redmine',
+    headers.reverse_merge! 'X-Mailer' => 'Redmine',
             'X-Redmine-Host' => Setting.host_name,
             'X-Redmine-Site' => Setting.app_title,
             'X-Auto-Response-Suppress' => 'OOF',