email replied id in references

application/Espo/Entities/Email.php

@@ -30,12 +30,11 @@
 namespace Espo\Entities;
 
 use Espo\Core\Utils\Util;
-
+use Espo\Core\ORM\Entity;
 use Espo\Entities\Attachment;
-
 use Espo\Services\Email as EmailService;
 
-class Email extends \Espo\Core\ORM\Entity
+class Email extends Entity
 {
     public const ENTITY_TYPE = 'Email';
 
@@ -343,4 +342,9 @@ class Email extends \Espo\Core\ORM\Entity
     {
         $this->set('messageId', 'dummy:' . Util::generateId());
     }
+
+    public function getMessageId(): ?string
+    {
+        return $this->get('messageId');
+    }
 }

application/Espo/Resources/metadata/entityAcl/Email.json

@@ -2,6 +2,9 @@
     "fields": {
         "users": {
             "readOnly": true
+        },
+        "messageId": {
+            "readOnly": true
         }
     },
     "links": {

application/Espo/Services/Email.php

@@ -284,6 +284,7 @@ class Email extends Record implements
 
         if ($repliedMessageId) {
             $message->getHeaders()->addHeaderLine('In-Reply-To', $repliedMessageId);
+            $message->getHeaders()->addHeaderLine('References', $repliedMessageId);
         }
 
         try {
@@ -980,6 +981,7 @@ class Email extends Record implements
             return null;
         }
 
+        /** @var EmailEntity $replied */
         $replied = $this->entityManager
             ->getRDBRepository('Email')
             ->select(['messageId'])
@@ -992,6 +994,6 @@ class Email extends Record implements
             return null;
         }
 
-        return $replied->get('messageId');
+        return $replied->getMessageId();
     }
 }