fix(auth): reject email domains with error website responses

service/email_domain_dns.go

@@ -19,7 +19,7 @@ type emailDomainDNSCheck struct {
 	MXHost           string // first MX host, lowercase, trailing dot trimmed
 	ARecord          bool   // at least one A/AAAA record resolves
 	PrivateARecord   bool   // resolved A points to loopback/private/link-local (parked placeholder)
-	WebsiteReachable bool   // an HTTP(S) server answered on port 80/443
+	WebsiteReachable bool   // an HTTP(S) server returned a non-error status on port 80/443
 	MajorProviderMX  bool   // MX host belongs to a mainstream mail provider (gmail/outlook/qq/...)
 	DisposableMX     bool   // MX host matches known disposable/temp-mail infrastructure
 }
@@ -265,8 +265,13 @@ func safeEmailDomainDialContext(ctx context.Context, network, addr string) (net.
 }
 
 // emailDomainWebsiteReachable returns true if an HTTP(S) server answers on the
-// domain (any status code counts — a real site exists). TLS failure falls back
-// to plain HTTP. Connections are restricted to public IPs (SSRF hardening).
+// domain with a non-error (2xx/3xx) status. 4xx/5xx responses mean the root
+// domain is unavailable for registration. TLS failure falls back to plain HTTP.
+// Connections are restricted to public IPs (SSRF hardening).
+func emailDomainWebsiteStatusOK(statusCode int) bool {
+	return statusCode >= http.StatusOK && statusCode < http.StatusBadRequest
+}
+
 func emailDomainWebsiteReachable(domain string) bool {
 	for _, scheme := range []string{"https", "http"} {
 		ctx, cancel := context.WithTimeout(context.Background(), 2500*time.Millisecond)
@@ -280,7 +285,7 @@ func emailDomainWebsiteReachable(domain string) bool {
 		cancel()
 		if err == nil {
 			resp.Body.Close()
-			return true
+			return emailDomainWebsiteStatusOK(resp.StatusCode)
 		}
 	}
 	return false

service/email_domain_dns_test.go

@@ -56,3 +56,12 @@ func TestSafeEmailDomainDialContextRefusesPrivateTargets(t *testing.T) {
 	_, err = safeEmailDomainDialContext(context.Background(), "tcp", "169.254.169.254:80")
 	require.Error(t, err)
 }
+
+func TestEmailDomainWebsiteStatusRequiresSuccessfulResponse(t *testing.T) {
+	for _, status := range []int{200, 204, 301, 302, 399} {
+		require.True(t, emailDomainWebsiteStatusOK(status), status)
+	}
+	for _, status := range []int{400, 401, 403, 404, 500, 502, 503} {
+		require.False(t, emailDomainWebsiteStatusOK(status), status)
+	}
+}

service/registration_security.go

@@ -51,7 +51,7 @@ func EvaluateRegistrationEmail(email string, cfg system_setting.RegistrationSecu
 			if cfg.RejectEmailDomainWithoutMX && !dnsCheck.MXRecord {
 				return RegistrationEmailDecision{}, ErrRegistrationDomainUnavailable
 			}
-			if cfg.RejectEmailDomainWithoutWebsite && !dnsCheck.WebsiteReachable && !dnsCheck.MajorProviderMX {
+			if cfg.RejectEmailDomainWithoutWebsite && !dnsCheck.WebsiteReachable {
 				return RegistrationEmailDecision{}, ErrRegistrationDomainUnavailable
 			}
 		}

service/registration_security_test.go

@@ -155,11 +155,11 @@ func TestRegistrationSecurityDNSMissingWebsitePolicy(t *testing.T) {
 	_, err = EvaluateRegistrationEmail("user@withweb.example", cfg, nil)
 	require.NoError(t, err)
 
-	// major-provider MX exempts from the website requirement (email-only domain)
+	// A major-provider MX does not exempt a domain whose root website is unavailable.
 	restore = stubDNSChecker(emailDomainDNSCheck{MXRecord: true, MXHost: "alt1.gmail-smtp-in.l.google.com", MajorProviderMX: true})
 	defer restore()
 	_, err = EvaluateRegistrationEmail("user@corp.example", cfg, nil)
-	require.NoError(t, err)
+	require.ErrorIs(t, err, ErrRegistrationDomainUnavailable)
 }
 
 func TestRegistrationSecurityDNSDisabledByDefaultInPolicy(t *testing.T) {