Add support for sender rewriting using postsrsd

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

mail-server/postfix.nix

@@ -261,6 +261,24 @@ in
       configurePostfix = true;
     };
 
+    # Sender Rewriting Scheme (https://www.libsrs2.net/srs/srs.pdf)
+    services.postsrsd = {
+      inherit (cfg.srs) enable;
+      configurePostfix = true;
+      settings = {
+        domains = lib.unique (
+          [
+            cfg.fqdn
+            cfg.sendingFqdn
+            cfg.systemDomain
+          ]
+          ++ cfg.domains
+        );
+        separator = "=";
+        srs-domain = cfg.srs.domain;
+      };
+    };
+
     systemd.services.postfix-setup = lib.mkIf cfg.ldap.enable {
       preStart = ''
         ${appendPwdInVirtualMailboxMap}