Add configuration option to alias entire domains and respective tests

mailsystem/postfix.nix

@@ -28,9 +28,18 @@ with (import ./common.nix {inherit config;}); let
       map (from: {"${from}" = to;}) (value.aliases ++ lib.singleton name))
     cfg.accounts));
 
+  virtual_domain_aliases = let
+    alias_domains =
+      lib.concatMapAttrs (src: dst: {
+        "@${src}" = "@${dst}";
+      })
+      cfg.virtualDomainAliases;
+  in
+    attrsToLookupTable alias_domains;
+
   extra_virtual_aliases = attrsToLookupTable cfg.extraVirtualAliases;
 
-  all_virtual_aliases = mergeLookupTables [account_virtual_aliases extra_virtual_aliases];
+  all_virtual_aliases = mergeLookupTables [account_virtual_aliases virtual_domain_aliases extra_virtual_aliases];
 
   aliases_file = let
     content = lookupTableToString all_virtual_aliases;
@@ -65,6 +74,15 @@ with (import ./common.nix {inherit config;}); let
   tls_exclude_ciphers = "MD5, DES, ADH, RC4, PSD, SRP, 3DES, eNULL, aNULL";
 in {
   config = lib.mkIf cfg.enable {
+    assertions =
+      lib.mapAttrsToList (
+        src: dst: {
+          assertion = (builtins.elem src cfg.domains) && (builtins.elem dst cfg.domains);
+          message = "Both aliased domain (${src}) and actual domain (${dst}) need to be managed by the mailserver.";
+        }
+      )
+      cfg.virtualDomainAliases;
+
     services.postfix = {
       enable = true;
       hostname = "${cfg.reverseFqdn}";