stalwart-mail: add v0.13.x breaking change notice

doc/release-notes/rl-2511.section.md

@@ -33,6 +33,8 @@
 
 - `conftest` since `0.60.0` has moved to use rego `v1` as default. To continue using `v0` use `--rego-version v0`. For more information about upgrading to Rego v1 syntax, see the [upstream docs](https://www.openpolicyagent.org/docs/latest/v0-upgrade/).
 
+- `stalwart-mail` since `0.13.0` "introduces a significant redesign of the MTA’s delivery and queueing subsystem". See [the upgrading announcement for the `0.13.0` release](https://github.com/stalwartlabs/stalwart/blob/89b561b5ca1c5a11f2a768b4a2cfef0f473b7a01/UPGRADING.md#upgrading-from-v012x-and-v011x-to-v013x).
+
 - The `archipelago-minecraft` package was removed, as upstream no longer provides support for the Minecraft APWorld.
 
 - `tooling-language-server` has been renamed to `deputy` (both the package and binary), following the rename of the upstream project.

nixos/modules/services/mail/stalwart-mail.nix

@@ -71,6 +71,23 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion =
+          !(
+            (lib.hasAttrByPath [ "settings" "queue" ] cfg)
+            && (builtins.any (lib.hasAttrByPath [
+              "value"
+              "next-hop"
+            ]) (lib.attrsToList cfg.settings.queue))
+          );
+        message = ''
+          Stalwart deprecated `next-hop` in favor of "virtual queues" `queue.strategy.route` \
+          with v0.13.0 see [Outbound Strategy](https://stalw.art/docs/mta/outbound/strategy/#configuration) \
+          and [release announcement](https://github.com/stalwartlabs/stalwart/blob/main/UPGRADING.md#upgrading-from-v012x-and-v011x-to-v013x).
+        '';
+      }
+    ];
 
     # Default config: all local
     services.stalwart-mail.settings = {