diff --git a/src/app/definition.h b/src/app/definition.h
index 3da8a76..a06618b 100644
--- a/src/app/definition.h
+++ b/src/app/definition.h
@@ -30,7 +30,7 @@ typedef enum
   MAILBOX_COLLECTED,
   MAILBOX_TEST,
   MAILBOX_UNKNOWN,
-} MAILBOX_EVENT_e;
+} __attribute__((__packed__)) MAILBOX_EVENT_e;
 
 typedef struct 
 {