Add `forwards` option

This option allow to forward mails to external addresses.

default.nix

@@ -218,6 +218,24 @@ in
       default = {};
     };
 
+    forwards = mkOption {
+      type = with types; attrsOf (either (listOf str) str);
+      default = {};
+      example = {
+        "user@example.com" = "user@elsewhere.com";
+      };
+      description = ''
+        To forward mails to an external address. For instance,
+        the value {`"user@example.com" = "user@elsewhere.com";}`
+        means that mails to `user@example.com` are forwarded to
+        `user@elsewhere.com`. The difference with the
+        `extraVirtualAliases` option is that `user@elsewhere.com`
+        can't send mail as `user@example.com`. Also, this option
+        allows to forward mails to external addresses.
+      '';
+      default = {};
+    };
+
     rejectSender = mkOption {
       type = types.listOf types.str;
       example = [ "@example.com" "spammer@example.net" ];

mail-server/postfix.nix

@@ -37,15 +37,7 @@ let
     cfg.loginAccounts);
 
   # extra_valiases_postfix :: [ String ]
-  extra_valiases_postfix =
+  extra_valiases_postfix = attrsToAliasList cfg.extraVirtualAliases;
-    (map
-    (from:
-      let to = cfg.extraVirtualAliases.${from};
-          aliasList = (l: let aliasStr = builtins.foldl' (x: y: x + y + ", ") "" l;
-                          in builtins.substring 0 (builtins.stringLength aliasStr - 2) aliasStr);
-      in if (builtins.isList to) then "${from} " + (aliasList to)
-                                 else "${from} ${to}")
-    (builtins.attrNames cfg.extraVirtualAliases));
 
   # all_valiases_postfix :: [ String ]
   all_valiases_postfix = valiases_postfix ++ extra_valiases_postfix;
@@ -56,6 +48,16 @@ let
     (lib.filter (acct: acct.sendOnly) (lib.attrValues cfg.loginAccounts)));
   denied_recipients_file = builtins.toFile "denied_recipients" (lib.concatStringsSep "\n" denied_recipients_postfix);
 
+  # attrsToAliasList :: Map String (Either String [ String ]) -> [ String ]
+  attrsToAliasList = aliases:
+    let
+      toList = to: if builtins.isList to then to else [to];
+    in lib.mapAttrsToList
+         (from: to: "${from} " + (lib.concatStringsSep ", " (toList to)))
+         aliases;
+
+  # forwards :: [ String ]
+  forwards = attrsToAliasList cfg.forwards;
 
   # valiases_file :: Path
   valiases_file = builtins.toFile "valias"
@@ -136,7 +138,7 @@ in
       sslKey = keyPath;
       enableSubmission = true;
       virtual =
-        (lib.concatStringsSep "\n" (all_valiases_postfix ++ catchAllPostfix));
+        (lib.concatStringsSep "\n" (all_valiases_postfix ++ catchAllPostfix ++ forwards));
 
       config = {
         # Extra Config