Add support for sender rewriting using postsrsd

With SRS we support forwarding of mails without (fully) breaking SPF
alignment.

default.nix

@@ -1101,6 +1101,28 @@ in
       '';
     };
 
+    srs = {
+      enable = mkEnableOption "Sender Rewrite Scheme";
+
+      domain = mkOption {
+        type = with types; nullOr str;
+        default = config.mailserver.systemDomain;
+        defaultText = literalExpression "config.mailserver.systemDomain";
+        example = "srs.example.com";
+        description = ''
+          Mail domain used for ephemeral SRS envelope addresses.
+
+          :::{note}
+          This domain can only support relaxed SPF alignment.
+          :::
+
+          :::{important}
+          For privacy reasons you should use a dedicated domain when serving multiple unrelated domains.
+          :::
+        '';
+      };
+    };
+
     redis = {
       configureLocally = mkOption {
         type = types.bool;