diff --git a/include/data_model.h b/include/data_model.h
index 5b825f8..eadf2d3 100644
--- a/include/data_model.h
+++ b/include/data_model.h
@@ -15,7 +15,8 @@ enum class RxRejectReason : uint8_t {
   InvalidMsgKind = 2,
   LengthMismatch = 3,
   DeviceIdMismatch = 4,
-  BatchIdMismatch = 5
+  BatchIdMismatch = 5,
+  UnknownSender = 6
 };
 
 struct FaultCounters {
diff --git a/src/data_model.cpp b/src/data_model.cpp
index 0ecdbe9..34b2241 100644
--- a/src/data_model.cpp
+++ b/src/data_model.cpp
@@ -21,6 +21,8 @@ const char *rx_reject_reason_text(RxRejectReason reason) {
       return "device_id_mismatch";
     case RxRejectReason::BatchIdMismatch:
       return "batch_id_mismatch";
+    case RxRejectReason::UnknownSender:
+      return "unknown_sender";
     default:
       return "none";
   }
diff --git a/src/main.cpp b/src/main.cpp
index fbceb70..a4c9bc9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -96,6 +96,8 @@ static uint32_t g_sender_sleep_ms = 0;
 static uint32_t g_sender_power_log_ms = 0;
 static RxRejectReason g_sender_rx_reject_reason = RxRejectReason::None;
 static uint32_t g_sender_rx_reject_log_ms = 0;
+static RxRejectReason g_receiver_rx_reject_reason = RxRejectReason::None;
+static uint32_t g_receiver_rx_reject_log_ms = 0;
 static MeterData g_last_meter_data = {};
 static bool g_last_meter_valid = false;
 static uint32_t g_last_meter_rx_ms = 0;
@@ -421,6 +423,18 @@ static void sender_note_rx_reject(RxRejectReason reason, const char *context) {
   }
 }
 
+static void receiver_note_rx_reject(RxRejectReason reason, const char *context) {
+  if (reason == RxRejectReason::None) {
+    return;
+  }
+  g_receiver_rx_reject_reason = reason;
+  uint32_t now_ms = millis();
+  if (SERIAL_DEBUG_MODE && now_ms - g_receiver_rx_reject_log_ms >= 1000) {
+    g_receiver_rx_reject_log_ms = now_ms;
+    serial_debug_printf("rx_reject: %s reason=%s", context, rx_reject_reason_text(reason));
+  }
+}
+
 static BatchBuffer *batch_queue_peek() {
   if (g_batch_count == 0) {
     return nullptr;
@@ -1419,30 +1433,51 @@ static void receiver_loop() {
           }
         }
 
-        bool duplicate = sender_idx >= 0 && g_last_batch_id_rx[sender_idx] == batch_id;
-        if (sender_idx >= 0) {
-          SenderStatus &status = g_sender_statuses[sender_idx];
-          if (status.rx_batches_total < UINT32_MAX) {
-            status.rx_batches_total++;
-          }
-          if (duplicate) {
-            if (status.rx_batches_duplicate < UINT32_MAX) {
-              status.rx_batches_duplicate++;
-            }
-            uint32_t duplicate_ts = time_get_utc();
-            if (duplicate_ts == 0) {
-              duplicate_ts = batch.t_last;
-            }
-            status.rx_last_duplicate_ts_utc = duplicate_ts;
-          }
+        if (sender_idx < 0) {
+          receiver_note_rx_reject(RxRejectReason::UnknownSender, "batch");
+          note_fault(g_receiver_faults, g_receiver_last_error, g_receiver_last_error_utc, g_receiver_last_error_ms, FaultType::Decode);
+          display_set_last_error(g_receiver_last_error, g_receiver_last_error_utc, g_receiver_last_error_ms);
+          serial_debug_printf("batch: reject unknown_sender short_id=%04X sender_id=%u batch_id=%u",
+                              pkt.device_id_short,
+                              static_cast<unsigned>(batch.sender_id),
+                              static_cast<unsigned>(batch_id));
+          goto receiver_loop_done;
         }
+
+        uint16_t expected_sender_id = static_cast<uint16_t>(sender_idx + 1);
+        if (batch.sender_id != expected_sender_id) {
+          receiver_note_rx_reject(RxRejectReason::DeviceIdMismatch, "batch");
+          note_fault(g_receiver_faults, g_receiver_last_error, g_receiver_last_error_utc, g_receiver_last_error_ms, FaultType::Decode);
+          display_set_last_error(g_receiver_last_error, g_receiver_last_error_utc, g_receiver_last_error_ms);
+          serial_debug_printf("batch: reject device_id_mismatch short_id=%04X sender_id=%u expected=%u batch_id=%u",
+                              pkt.device_id_short,
+                              static_cast<unsigned>(batch.sender_id),
+                              static_cast<unsigned>(expected_sender_id),
+                              static_cast<unsigned>(batch_id));
+          goto receiver_loop_done;
+        }
+
+        bool duplicate = g_last_batch_id_rx[sender_idx] == batch_id;
+        SenderStatus &status = g_sender_statuses[sender_idx];
+        if (status.rx_batches_total < UINT32_MAX) {
+          status.rx_batches_total++;
+        }
+        if (duplicate) {
+          if (status.rx_batches_duplicate < UINT32_MAX) {
+            status.rx_batches_duplicate++;
+          }
+          uint32_t duplicate_ts = time_get_utc();
+          if (duplicate_ts == 0) {
+            duplicate_ts = batch.t_last;
+          }
+          status.rx_last_duplicate_ts_utc = duplicate_ts;
+        }
+
         send_batch_ack(batch_id, batch.n);
         if (duplicate) {
           goto receiver_loop_done;
         }
-        if (sender_idx >= 0) {
-          g_last_batch_id_rx[sender_idx] = batch_id;
-        }
+        g_last_batch_id_rx[sender_idx] = batch_id;
         if (batch.n == 0) {
           goto receiver_loop_done;
         }
@@ -1520,26 +1555,24 @@ static void receiver_loop() {
           goto receiver_loop_done;
         }
 
-        if (sender_idx >= 0) {
-          web_server_set_last_batch(static_cast<uint8_t>(sender_idx), samples, count);
-          for (size_t s = 0; s < count; ++s) {
-            mqtt_publish_state(samples[s]);
-          }
-          g_sender_statuses[sender_idx].last_data = samples[count - 1];
-          g_sender_statuses[sender_idx].last_update_ts_utc = samples[count - 1].ts_utc;
-          g_sender_statuses[sender_idx].has_data = true;
-          g_sender_faults_remote[sender_idx].meter_read_fail = samples[count - 1].err_meter_read;
-          g_sender_faults_remote[sender_idx].lora_tx_fail = samples[count - 1].err_lora_tx;
-          g_sender_last_error_remote[sender_idx] = samples[count - 1].last_error;
-          g_sender_last_error_remote_utc[sender_idx] = time_get_utc();
-          g_sender_last_error_remote_ms[sender_idx] = millis();
-          if (ENABLE_HA_DISCOVERY && !g_sender_discovery_sent[sender_idx]) {
-            g_sender_discovery_sent[sender_idx] = mqtt_publish_discovery(samples[count - 1].device_id);
-          }
-          publish_faults_if_needed(samples[count - 1].device_id, g_sender_faults_remote[sender_idx], g_sender_faults_remote_published[sender_idx],
-                                   g_sender_last_error_remote[sender_idx], g_sender_last_error_remote_published[sender_idx],
-                                   g_sender_last_error_remote_utc[sender_idx], g_sender_last_error_remote_ms[sender_idx]);
+        web_server_set_last_batch(static_cast<uint8_t>(sender_idx), samples, count);
+        for (size_t s = 0; s < count; ++s) {
+          mqtt_publish_state(samples[s]);
         }
+        g_sender_statuses[sender_idx].last_data = samples[count - 1];
+        g_sender_statuses[sender_idx].last_update_ts_utc = samples[count - 1].ts_utc;
+        g_sender_statuses[sender_idx].has_data = true;
+        g_sender_faults_remote[sender_idx].meter_read_fail = samples[count - 1].err_meter_read;
+        g_sender_faults_remote[sender_idx].lora_tx_fail = samples[count - 1].err_lora_tx;
+        g_sender_last_error_remote[sender_idx] = samples[count - 1].last_error;
+        g_sender_last_error_remote_utc[sender_idx] = time_get_utc();
+        g_sender_last_error_remote_ms[sender_idx] = millis();
+        if (ENABLE_HA_DISCOVERY && !g_sender_discovery_sent[sender_idx]) {
+          g_sender_discovery_sent[sender_idx] = mqtt_publish_discovery(samples[count - 1].device_id);
+        }
+        publish_faults_if_needed(samples[count - 1].device_id, g_sender_faults_remote[sender_idx], g_sender_faults_remote_published[sender_idx],
+                                 g_sender_last_error_remote[sender_idx], g_sender_last_error_remote_published[sender_idx],
+                                 g_sender_last_error_remote_utc[sender_idx], g_sender_last_error_remote_ms[sender_idx]);
       } else if (decode_error) {
         note_fault(g_receiver_faults, g_receiver_last_error, g_receiver_last_error_utc, g_receiver_last_error_ms, FaultType::Decode);
         display_set_last_error(g_receiver_last_error, g_receiver_last_error_utc, g_receiver_last_error_ms);
diff --git a/src/web_server.cpp b/src/web_server.cpp
index c68dbb3..6fadec3 100644
--- a/src/web_server.cpp
+++ b/src/web_server.cpp
@@ -698,7 +698,7 @@ static void handle_manual() {
   html += "<li>RSSI/SNR: LoRa link quality from last packet.</li>";
   html += "<li>err_tx: sender-side LoRa TX error counter.</li>";
   html += "<li>err_last: last error code (0=None, 1=MeterRead, 2=Decode, 3=LoraTx).</li>";
-  html += "<li>rx_reject: last RX reject reason (0=None, 1=crc_fail, 2=invalid_msg_kind, 3=length_mismatch, 4=device_id_mismatch, 5=batch_id_mismatch).</li>";
+  html += "<li>rx_reject: last RX reject reason (0=None, 1=crc_fail, 2=invalid_msg_kind, 3=length_mismatch, 4=device_id_mismatch, 5=batch_id_mismatch, 6=unknown_sender).</li>";
   html += "<li>faults m/d/tx: receiver-side counters (meter read fails, decode fails, LoRa TX fails).</li>";
   html += "<li>faults last: last receiver-side error code (same mapping as err_last).</li>";
   html += "</ul>";