diff --git a/Kconfig b/Kconfig
index 2c274ca..9eb6c0b 100644
--- a/Kconfig
+++ b/Kconfig
@@ -152,6 +152,13 @@ menu "ESP-MQTT Configurations"
             bool "Core 1"
     endchoice
 
+    config MQTT_OUTBOX_DATA_ON_EXTERNAL_MEMORY
+        bool "Use external memory for outbox data"
+        default n
+        depends on MQTT_USE_CUSTOM_CONFIG
+        help
+            Set to true to use external memory for outbox data.
+
     config MQTT_CUSTOM_OUTBOX
         bool "Enable custom outbox implementation"
         default n
diff --git a/lib/include/mqtt_config.h b/lib/include/mqtt_config.h
index 6238ee1..409fb3c 100644
--- a/lib/include/mqtt_config.h
+++ b/lib/include/mqtt_config.h
@@ -19,10 +19,10 @@
 #define MQTT_RECON_DEFAULT_MS       (10*1000)
 
 #ifdef CONFIG_MQTT_POLL_READ_TIMEOUT_MS
-#define MQTT_POLL_READ_TIMEOUT_MS  CONFIG_MQTT_POLL_READ_TIMEOUT_MS 
+#define MQTT_POLL_READ_TIMEOUT_MS  CONFIG_MQTT_POLL_READ_TIMEOUT_MS
 #else
 #define MQTT_POLL_READ_TIMEOUT_MS   (1000)
-#endif 
+#endif
 
 #define MQTT_MSG_ID_INCREMENTAL     CONFIG_MQTT_MSG_ID_INCREMENTAL
 
@@ -107,5 +107,11 @@
 #define MQTT_EVENT_QUEUE_SIZE       1
 #endif
 
+#ifdef CONFIG_MQTT_OUTBOX_DATA_ON_EXTERNAL_MEMORY
+#define MQTT_OUTBOX_MEMORY MALLOC_CAP_SPIRAM
+#else
+#define MQTT_OUTBOX_MEMORY MALLOC_CAP_DEFAULT
+#endif
+
 #define OUTBOX_MAX_SIZE             (4*1024)
 #endif
diff --git a/lib/mqtt_outbox.c b/lib/mqtt_outbox.c
index 5680e8a..f60a471 100644
--- a/lib/mqtt_outbox.c
+++ b/lib/mqtt_outbox.c
@@ -1,7 +1,9 @@
 #include "mqtt_outbox.h"
 #include <stdlib.h>
 #include <string.h>
+#include "mqtt_config.h"
 #include "sys/queue.h"
+#include "esp_heap_caps.h"
 #include "esp_log.h"
 
 #ifndef CONFIG_MQTT_CUSTOM_OUTBOX
@@ -31,7 +33,7 @@ outbox_handle_t outbox_init(void)
 
 outbox_item_handle_t outbox_enqueue(outbox_handle_t outbox, outbox_message_handle_t message, outbox_tick_t tick)
 {
-    outbox_item_handle_t item = calloc(1, sizeof(outbox_item_t));
+    outbox_item_handle_t item = heap_caps_calloc(1, sizeof(outbox_item_t), MQTT_OUTBOX_MEMORY);
     ESP_MEM_CHECK(TAG, item, return NULL);
     item->msg_id = message->msg_id;
     item->msg_type = message->msg_type;