fix: join confirmation mail configuration

test/membership/join_request_submit_email_failure_test.exs

@@ -0,0 +1,33 @@
+defmodule Mv.Membership.JoinRequestSubmitEmailFailureTest do
+  @moduledoc """
+  Tests that when join confirmation email delivery fails, the domain returns
+  {:error, :email_delivery_failed} (and the LiveView shows an error). Uses
+  FailingMailAdapter to simulate delivery failure; async: false to avoid config races.
+  """
+  use Mv.DataCase, async: false
+
+  alias Mv.Membership
+
+  @valid_submit_attrs %{
+    email: "fail#{System.unique_integer([:positive])}@example.com"
+  }
+
+  test "submit_join_request returns {:error, :email_delivery_failed} when mail delivery fails" do
+    saved = Application.get_env(:mv, Mv.Mailer)
+
+    Application.put_env(
+      :mv,
+      Mv.Mailer,
+      Keyword.put(saved || [], :adapter, Mv.TestSupport.FailingMailAdapter)
+    )
+
+    on_exit(fn ->
+      Application.put_env(:mv, Mv.Mailer, saved)
+    end)
+
+    token = "fail-token-#{System.unique_integer([:positive])}"
+    attrs = Map.put(@valid_submit_attrs, :confirmation_token, token)
+
+    assert {:error, :email_delivery_failed} = Membership.submit_join_request(attrs, actor: nil)
+  end
+end