diff --git a/roles/mail_server/files/ferm_mail.conf b/roles/mail_server/files/ferm_mail.conf
index 3d95b6d19e1b1cc010ee92732188a26beb462d6b..0c2cc789c8a6ff35c1b3d0a996d34173b5f72dcb 100644
--- a/roles/mail_server/files/ferm_mail.conf
+++ b/roles/mail_server/files/ferm_mail.conf
@@ -16,9 +16,12 @@ domain (ip ip6) {
 
     table nat {
         chain PREROUTING {
-            # Set-up redirection for alternate SMTP port (to avoid ISP/hotel blocks
-            # etc).
+            # Set-up redirection for alternate SMTP submission port (to avoid
+            # ISP/hotel blocks etc).
             proto tcp dport 26 REDIRECT to-ports 587;
+
+            # Set-up redirection for alternate SMTP port (to avoid ISP blocks).
+            proto tcp dport 27 REDIRECT to-ports 25;
         }
     }
 }
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