add link to mail-tester.com in test email

config/locales/server.en.yml

@@ -1485,6 +1485,8 @@ en:
 
       - If you run your own mail server, check to make sure the IPs of your mail server are [not on any email blacklists][4]. Also verify that it is definitely sending a fully-qualified hostname that resolves in DNS in its HELO message. If not, this will cause your email to be rejected by many mail services.
 
+      - Send a test email to [mail-tester.com][mt] to verify that everything is working correctly. 
+
       (The *easy* way is to create a free account on [Mandrill][md] or [Mailgun][mg] or [Mailjet][mj], which have free generous free mailing plans and will be fine for small communities. You'll still need to set up the SPF and DKIM records in your DNS, though!)
 
       We hope you received this email deliverability test OK!
@@ -1503,6 +1505,7 @@ en:
       [md]: http://mandrill.com
       [mg]: http://www.mailgun.com/
       [mj]: https://www.mailjet.com/pricing
+      [mt]: http://www.mail-tester.com/
 
   new_version_mailer:
     subject_template: "[%{site_name}] New Discourse version, update available"