Add support for sender rewriting using postsrsd

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

mail-server/rspamd.nix

@@ -50,6 +50,8 @@ let
         echo "Generated key for domain ${domain} and selector ${cfg.dkimSelector}"
       fi
     '';
+
+  dkimDomains = lib.unique (cfg.domains ++ (lib.optionals cfg.srs.enable [ cfg.srs.domain ]));
 in
 {
   config = lib.mkIf cfg.enable {
@@ -201,7 +203,7 @@ in
           SupplementaryGroups = [ config.services.redis.servers.rspamd.group ];
         }
         (lib.optionalAttrs cfg.dkimSigning {
-          ExecStartPre = map createDkimKeypair cfg.domains;
+          ExecStartPre = map createDkimKeypair dkimDomains;
           ReadWritePaths = [ cfg.dkimKeyDirectory ];
         })
       ];