diff --git a/twe/j_msgsubmit.php b/twe/j_msgsubmit.php
index b28ac316..3545e2e2 100644
--- a/twe/j_msgsubmit.php
+++ b/twe/j_msgsubmit.php
@@ -131,7 +131,14 @@ if($destMailbox == 9999) {
     }
 
     sendMessage('national', $src, $dest, $msg, $date);
-
+    
+} elseif($destMailbox >= 8000){
+    //외교 서신은 j_msgsubmit으로 보낼 수 없음.
+    returnJson([
+        'result' => false,
+        'reason' => '존재하지 않는 유저입니다.',
+        'redirect' => NULL
+    ]);
 // 개인 메세지
 } elseif($destMailbox > 0) {
     $last_msg = new DateTime(util::array_get($_SESSION['last_msg'], '0000-00-00'));
diff --git a/twe/schema.php b/twe/schema.php
index ca2ebe36..7a764e65 100644
--- a/twe/schema.php
+++ b/twe/schema.php
@@ -534,7 +534,7 @@ $game_schema = "
 $message_schema = "
 CREATE TABLE `message` (
 	`id` INT(11) NOT NULL AUTO_INCREMENT,
-	`mailbox` INT(11) NOT NULL COMMENT '9999 == public, >= 9000 national',
+	`mailbox` INT(11) NOT NULL COMMENT '9999 == public, >= 9000 national, >= 8000 diplomacy',
 	`type` ENUM('private','national','public','diplomacy') NOT NULL,
 	`is_sender` BIT(1) NOT NULL,
 	`src` INT(11) NOT NULL,