diff --git a/application/Espo/Core/Mail/SmtpParams.php b/application/Espo/Core/Mail/SmtpParams.php
index 4ba80a5a51..c696b7746e 100644
--- a/application/Espo/Core/Mail/SmtpParams.php
+++ b/application/Espo/Core/Mail/SmtpParams.php
@@ -69,6 +69,7 @@ class SmtpParams
         'username',
         'password',
         'security',
+        'transportPreparatorClassName',
     ];
 
     public function __construct(