diff --git a/mail-server.nix b/mail-server.nix
index 6a854d7..544ae40 100644
--- a/mail-server.nix
+++ b/mail-server.nix
@@ -75,6 +75,12 @@ in {
         description = "Organizational unit containing users.";
         default = "ou=members";
       };
+
+      metricsPort = mkOption {
+        type = port;
+        description = "Port on which to serve metrics.";
+        default = 5457;
+      };
     };
 
     images.ldap-proxy = mkOption {