diff --git a/nixos/modules/services/mail/rspamd.nix b/nixos/modules/services/mail/rspamd.nix
index 473ddd52357..c78f464235a 100644
--- a/nixos/modules/services/mail/rspamd.nix
+++ b/nixos/modules/services/mail/rspamd.nix
@@ -371,8 +371,9 @@ in
     };
     services.postfix.config = mkIf cfg.postfix.enable cfg.postfix.config;
 
-    systemd.services.postfix.serviceConfig.SupplementaryGroups =
-      mkIf cfg.postfix.enable [ postfixCfg.group ];
+    systemd.services.postfix = mkIf cfg.postfix.enable {
+      serviceConfig.SupplementaryGroups = [ postfixCfg.group ];
+    };
 
     # Allow users to run 'rspamc' and 'rspamadm'.
     environment.systemPackages = [ pkgs.rspamd ];