diff --git a/lib/__tests__/proxy-security-headers.test.ts b/lib/__tests__/proxy-security-headers.test.ts
index 97be8c632..d1837a8a2 100644
--- a/lib/__tests__/proxy-security-headers.test.ts
+++ b/lib/__tests__/proxy-security-headers.test.ts
@@ -135,6 +135,19 @@ describe('ALLOWED_FRAME_ANCESTORS', () => {
     else process.env.ALLOWED_FRAME_ANCESTORS = original;
   });
 
+  it('sends HSTS only on a request that arrived over TLS', async () => {
+    const plain = await load('/en/mail');
+    expect(plain.headers.get('strict-transport-security')).toBeNull();
+    const forwarded = await proxy(new NextRequest('http://localhost:3000/en/mail', {
+      headers: { 'sec-fetch-dest': 'document', 'sec-fetch-mode': 'navigate', 'sec-fetch-site': 'none', 'x-forwarded-proto': 'https' },
+    }));
+    expect(forwarded.headers.get('strict-transport-security')).toBe('max-age=63072000');
+    const direct = await proxy(new NextRequest('https://webmail.example/en/mail', {
+      headers: { 'sec-fetch-dest': 'document', 'sec-fetch-mode': 'navigate', 'sec-fetch-site': 'none' },
+    }));
+    expect(direct.headers.get('strict-transport-security')).toBe('max-age=63072000');
+  });
+
   it('lets a portal frame the mail UI', async () => {
     const res = await load('/en/mail');
     expect(res.headers.get('content-security-policy')).toContain('frame-ancestors https://portal.example');
diff --git a/next.config.ts b/next.config.ts
index 65f3e6c1b..ca4915334 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -49,6 +49,8 @@ if (isLite && (process.env.NEXT_PUBLIC_LOCALE_PREFIX ?? "never") !== "always") {
 
 const nextConfig: NextConfig = {
   output: isLite ? "export" : "standalone",
+  // No `X-Powered-By: Next.js` on every response.
+  poweredByHeader: false,
   // Directory-style pages (out/en/mail/index.html) are what static hosts and
   // the shipped SPA-fallback rules expect; next/image has no optimizer here.
   ...(isLite ? { trailingSlash: true, images: { unoptimized: true } } : {}),
diff --git a/proxy.ts b/proxy.ts
index f00047d52..722b2457d 100644
--- a/proxy.ts
+++ b/proxy.ts
@@ -398,6 +398,13 @@ export async function proxy(request: NextRequest) {
 
   response.headers.set("Referrer-Policy", "strict-origin-when-cross-origin");
   response.headers.set("X-XSS-Protection", "0");
+  // Only on a request that arrived over TLS (directly or via the reverse
+  // proxy's X-Forwarded-Proto): a plain-http dev or LAN install must not be
+  // pinned to https by a header it cannot honour. No includeSubDomains, which
+  // would speak for the operator's other hosts.
+  if (request.nextUrl.protocol === "https:" || request.headers.get("x-forwarded-proto") === "https") {
+    response.headers.set("Strict-Transport-Security", "max-age=63072000");
+  }
   response.headers.set(
     "Permissions-Policy",
     "camera=(), microphone=(), geolocation=(), payment=()"