default.nix, mail-server/users.nix: add per-user sieve script

default.nix

@@ -59,6 +59,29 @@ in
               ```
             '';
           };
+
+          sieveScript = mkOption {
+            type = with types; nullOr lines;
+            default = null;
+            example = ''
+              require ["fileinto", "mailbox"];
+
+              if address :is "from" "notifications@github.com" {
+                fileinto :create "GitHub";
+                stop;
+              }
+
+              # This must be the last rule, it will check if list-id is set, and
+              # file the message into the Lists folder for further investigation
+              elsif header :matches "list-id" "<?*>" {
+                fileinto :create "Lists";
+                stop;
+              }
+            '';
+            description = ''
+              Per-user sieve script.
+            '';
+          };
         };
 
         config.name = mkDefault name;