host(kita): initial postfix configuration

systems/kita/services/mail/default.nix

@@ -1,5 +1,8 @@
 {...}: {
   imports = [
     ./roundcube.nix
+
+    # HELL
+    ./postfix.nix
   ];
 }

systems/kita/services/mail/postfix.nix

@@ -0,0 +1,30 @@
+{config, ...}: let
+  cfg = config.services.postfix;
+in {
+  services.postfix = {
+    enable = true;
+    domain = "soopy.moe";
+    hostname = "mx2.soopy.moe";
+    origin = "mx2.soopy.moe";
+    destination = [
+      "localhost"
+      "localhost.localdomain"
+      cfg.hostname
+      "staging.soopy.moe" # TODO: remove after testing
+      # don't put soopy.moe here, that goes into virtual stuff.
+    ];
+
+    # leave relayHost to be "".
+    relayDomains = [];
+    networksStyle = "host";
+
+    postmasterAlias = "cassie";
+    rootAlias = "cassie";
+  };
+
+  networking.firewall.allowedTCPPorts = [
+    25 # smtp
+    465 # submissions (secure)
+    # 587 # submission (starttls)
+  ];
+}