temmie/userweb: set up sendmail wrapper

hosts/temmie/services/userweb.nix

@@ -64,6 +64,21 @@ let
     ignoreCollisions = true;
   };
 
+  sendmailWrapper = pkgs.writeShellApplication {
+    name = "sendmail";
+    runtimeInputs = [ ];
+    text = ''
+      args=("$@")
+
+      if [[ "''${PWD:-}" =~ ^/home/pvv/[^/]+/([^/]+) ]] && [[ "''${BASH_REMATCH[1]}" != "pvv" ]]; then
+          # Prepend -fusername to the argument list, so bounces go to the user
+          args=("-f''${BASH_REMATCH[1]}" "''${args[@]}")
+      fi
+
+      exec '${lib.getExe pkgs.system-sendmail}' "''${args[@]}"
+    '';
+  };
+
   # https://nixos.org/manual/nixpkgs/stable/#sec-building-environment
   fhsEnv = pkgs.buildEnv {
     name = "userweb-env";
@@ -71,6 +86,8 @@ let
     paths = with pkgs; [
       bash
 
+      sendmailWrapper
+
       perlEnv
       pythonEnv
       phpEnv