diff --git a/nixos/modules/services/mail/rmilter.nix b/nixos/modules/services/mail/rmilter.nix
index c3e3d823242..0d91b247cd3 100644
--- a/nixos/modules/services/mail/rmilter.nix
+++ b/nixos/modules/services/mail/rmilter.nix
@@ -193,6 +193,9 @@ in
   config = mkMerge [
 
     (mkIf cfg.enable {
+      warnings = [
+        ''`config.services.rmilter' is deprecated, `rmilter' deprecated and unsupported by upstream, and will be removed from next releases. Use built-in rspamd milter instead.''
+      ];
 
       users.users = singleton {
         name = cfg.user;