host/mail-2: add host

flake.nix

@@ -41,6 +41,7 @@
       gatekeeper = { name = "gatekeeper"; };
       hydra-1 = { name = "hydra-1"; };
       hydra-2 = { name = "hydra-2"; };
+      mail-2 = { name = "mail-2"; };
       minecraft-2 = { name = "minecraft-2"; };
       monitoring-3 = { name = "monitoring-3"; };
       nonat = { name = "nonat"; };