diff --git a/messages/de.json b/messages/de.json
index 3a48fb6f1..b5ac510ae 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -6136,6 +6136,7 @@
         "rate_limited": "Vom Upstream rate-limitiert",
         "timeout": "Request hat das Zeitlimit überschritten",
         "upstream_error": "Upstream hat einen Fehler gemeldet",
+        "upstream_rejected": "Die Gegenstelle hat die Nachricht abgelehnt",
         "connection_failed": "Verbindung fehlgeschlagen",
         "not_configured": "Nicht konfiguriert",
         "url_not_public": "URL ist kein öffentlicher Endpunkt",
diff --git a/messages/en.json b/messages/en.json
index b2a37305f..e54cffb55 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -6136,6 +6136,7 @@
         "rate_limited": "Rate-limited by the upstream",
         "timeout": "Request timed out",
         "upstream_error": "Upstream returned an error",
+        "upstream_rejected": "The other end refused the message",
         "connection_failed": "Connection failed",
         "not_configured": "Not configured",
         "url_not_public": "URL is not a public endpoint",
diff --git a/messages/es.json b/messages/es.json
index e3f4ca56c..1ca650b2a 100644
--- a/messages/es.json
+++ b/messages/es.json
@@ -6136,6 +6136,7 @@
         "rate_limited": "Limitado por el servidor remoto",
         "timeout": "La solicitud agotó el tiempo de espera",
         "upstream_error": "El servidor remoto devolvió un error",
+        "upstream_rejected": "El servidor remoto rechazó el mensaje",
         "connection_failed": "Falló la conexión",
         "not_configured": "No configurado",
         "url_not_public": "La URL no es un endpoint público",
diff --git a/messages/fr.json b/messages/fr.json
index 48fc47a50..aba616f1d 100644
--- a/messages/fr.json
+++ b/messages/fr.json
@@ -6136,6 +6136,7 @@
         "rate_limited": "Limité par le service en amont",
         "timeout": "Délai de la requête dépassé",
         "upstream_error": "Le service en amont a renvoyé une erreur",
+        "upstream_rejected": "Le service distant a refusé le message",
         "connection_failed": "Échec de la connexion",
         "not_configured": "Non configuré",
         "url_not_public": "L’URL n’est pas un point de terminaison public",
diff --git a/messages/it.json b/messages/it.json
index 0e97ed7be..834d5da3d 100644
--- a/messages/it.json
+++ b/messages/it.json
@@ -6136,6 +6136,7 @@
         "rate_limited": "Limitato dal servizio upstream",
         "timeout": "Richiesta scaduta",
         "upstream_error": "Il servizio upstream ha restituito un errore",
+        "upstream_rejected": "Il servizio remoto ha rifiutato il messaggio",
         "connection_failed": "Connessione non riuscita",
         "not_configured": "Non configurato",
         "url_not_public": "L’URL non è un endpoint pubblico",
diff --git a/messages/ko.json b/messages/ko.json
index d09db31aa..48722e917 100644
--- a/messages/ko.json
+++ b/messages/ko.json
@@ -6136,6 +6136,7 @@
         "rate_limited": "상대 서버에서 요청 수를 제한했어요",
         "timeout": "요청 시간이 초과됐어요",
         "upstream_error": "상대 서버가 오류를 반환했어요",
+        "upstream_rejected": "상대 서버가 메시지를 거부했어요",
         "connection_failed": "연결에 실패했어요",
         "not_configured": "설정되지 않았어요",
         "url_not_public": "URL이 공개 엔드포인트가 아니에요",
diff --git a/messages/pl.json b/messages/pl.json
index 3c84cbc85..4689c536b 100644
--- a/messages/pl.json
+++ b/messages/pl.json
@@ -6136,6 +6136,7 @@
         "rate_limited": "Ograniczone przez usługę nadrzędną",
         "timeout": "Przekroczono limit czasu żądania",
         "upstream_error": "Usługa nadrzędna zwróciła błąd",
+        "upstream_rejected": "Usługa docelowa odrzuciła wiadomość",
         "connection_failed": "Połączenie nie powiodło się",
         "not_configured": "Nieskonfigurowano",
         "url_not_public": "Adres URL nie jest publicznym punktem końcowym",
diff --git a/src/components/settings/__tests__/test-connection-failure.test.tsx b/src/components/settings/__tests__/test-connection-failure.test.tsx
new file mode 100644
index 000000000..2002a70ac
--- /dev/null
+++ b/src/components/settings/__tests__/test-connection-failure.test.tsx
@@ -0,0 +1,61 @@
+/**
+ * The test-connection failure callout (#947).
+ *
+ * SSR-only, like the rest of the component suite: the callout is hook-free so
+ * it renders here without a provider. What is pinned is what a person reads:
+ * the translated sentence, the status the other end answered, and the relay's
+ * own words as text, never as markup.
+ */
+import { renderToStaticMarkup } from "react-dom/server";
+import { describe, expect, it } from "vitest";
+
+import { TestConnectionFailure } from "../test-connection-button";
+
+describe("TestConnectionFailure", () => {
+  it("shows the sentence with the HTTP status and what the relay said", () => {
+    const html = renderToStaticMarkup(
+      <TestConnectionFailure
+        message="The other end refused the message"
+        upstreamStatus={400}
+        upstreamBody='{"error":"Bad Request","errorCode":400}'
+      />,
+    );
+
+    expect(html).toContain('role="alert"');
+    expect(html).toContain("The other end refused the message (HTTP 400)");
+    expect(html).toContain(
+      "{&quot;error&quot;:&quot;Bad Request&quot;,&quot;errorCode&quot;:400}",
+    );
+  });
+
+  it("renders a markup-shaped body as characters", () => {
+    const html = renderToStaticMarkup(
+      <TestConnectionFailure
+        message="Upstream returned an error"
+        upstreamStatus={502}
+        upstreamBody='<img src=x onerror="alert(1)">'
+      />,
+    );
+
+    expect(html).not.toContain("<img");
+    expect(html).toContain("&lt;img src=x onerror=&quot;alert(1)&quot;&gt;");
+  });
+
+  it("shows the SMTP code when there is no HTTP status", () => {
+    const html = renderToStaticMarkup(
+      <TestConnectionFailure message="Credentials rejected" smtpCode={535} />,
+    );
+
+    expect(html).toContain("Credentials rejected (SMTP 535)");
+  });
+
+  it("shows the sentence alone when the server sent no detail", () => {
+    const html = renderToStaticMarkup(
+      <TestConnectionFailure message="Test failed" />,
+    );
+
+    expect(html).toContain("Test failed");
+    expect(html).not.toContain("HTTP");
+    expect(html).not.toContain("test-connection-upstream-body");
+  });
+});
diff --git a/src/components/settings/test-connection-button.tsx b/src/components/settings/test-connection-button.tsx
index 503b53079..e741a6233 100644
--- a/src/components/settings/test-connection-button.tsx
+++ b/src/components/settings/test-connection-button.tsx
@@ -11,7 +11,10 @@ import { apiFetchRaw } from "@/lib/api/api-fetch";
  * Shared "Test connection" UX for the settings integrations + notifications
  * sections (A8-UI). POSTs to the given endpoint, expects a `{ data, error,
  * meta }` envelope, and surfaces success (latency) or a translated
- * `meta.errorCode` callout.
+ * `meta.errorCode` callout. When the route reports what the other end
+ * answered (`meta.upstreamStatus` / `meta.smtpCode` / `meta.upstreamBody`,
+ * the notification test routes since #947), the callout shows the status and
+ * the relay's own words beneath it, as plain text.
  *
  * Endpoints are user-scoped, so the button intentionally does NOT send an
  * Idempotency-Key — each click probes the upstream live.
@@ -27,7 +30,56 @@ export interface TestConnectionButtonProps {
 interface TestResponse {
   data?: { latencyMs?: number; ok?: boolean; sent?: number };
   error?: string;
-  meta?: { errorCode?: string };
+  meta?: {
+    errorCode?: string;
+    upstreamStatus?: number;
+    smtpCode?: number;
+    upstreamBody?: string;
+  };
+}
+
+export interface TestConnectionFailureProps {
+  /** The translated error sentence. */
+  message: string;
+  upstreamStatus?: number;
+  smtpCode?: number;
+  /** What the other end answered, already bounded and cleaned by the server. */
+  upstreamBody?: string;
+}
+
+/**
+ * The failure callout. Hook-free so it renders without a provider. The
+ * relay's body is a React text child, never markup: whatever it contains is
+ * shown as characters.
+ */
+export function TestConnectionFailure({
+  message,
+  upstreamStatus,
+  smtpCode,
+  upstreamBody,
+}: TestConnectionFailureProps) {
+  const code =
+    typeof upstreamStatus === "number"
+      ? ` (HTTP ${upstreamStatus})`
+      : typeof smtpCode === "number"
+        ? ` (SMTP ${smtpCode})`
+        : "";
+  return (
+    <div role="alert" className="space-y-1">
+      <p className="text-destructive flex items-center gap-1.5 text-sm">
+        <XCircle className="size-3.5 shrink-0" />
+        {`${message}${code}`}
+      </p>
+      {upstreamBody ? (
+        <p
+          data-testid="test-connection-upstream-body"
+          className="text-foreground font-mono text-xs break-all"
+        >
+          {upstreamBody}
+        </p>
+      ) : null}
+    </div>
+  );
 }
 
 export function TestConnectionButton({
@@ -39,7 +91,13 @@ export function TestConnectionButton({
   const [testing, setTesting] = useState(false);
   const [result, setResult] = useState<
     | { kind: "ok"; latency: number }
-    | { kind: "error"; errorCode: string }
+    | {
+        kind: "error";
+        errorCode: string;
+        upstreamStatus?: number;
+        smtpCode?: number;
+        upstreamBody?: string;
+      }
     | null
   >(null);
 
@@ -56,9 +114,20 @@ export function TestConnectionButton({
           latency: json.data?.latencyMs ?? 0,
         });
       } else {
+        const meta = json.meta;
         setResult({
           kind: "error",
-          errorCode: json.meta?.errorCode ?? "generic",
+          errorCode: meta?.errorCode ?? "generic",
+          upstreamStatus:
+            typeof meta?.upstreamStatus === "number"
+              ? meta.upstreamStatus
+              : undefined,
+          smtpCode:
+            typeof meta?.smtpCode === "number" ? meta.smtpCode : undefined,
+          upstreamBody:
+            typeof meta?.upstreamBody === "string"
+              ? meta.upstreamBody
+              : undefined,
         });
       }
     } catch {
@@ -109,13 +178,12 @@ export function TestConnectionButton({
       )}
 
       {result?.kind === "error" && (
-        <p
-          role="alert"
-          className="text-destructive flex items-center gap-1.5 text-sm"
-        >
-          <XCircle className="h-3.5 w-3.5" />
-          {describeError(result.errorCode)}
-        </p>
+        <TestConnectionFailure
+          message={describeError(result.errorCode)}
+          upstreamStatus={result.upstreamStatus}
+          smtpCode={result.smtpCode}
+          upstreamBody={result.upstreamBody}
+        />
       )}
     </div>
   );