Index: branches/5.1.x/core/units/helpers/email_message_helper.php
===================================================================
diff -u -N -r13276 -r13811
--- branches/5.1.x/core/units/helpers/email_message_helper.php	(.../email_message_helper.php)	(revision 13276)
+++ branches/5.1.x/core/units/helpers/email_message_helper.php	(.../email_message_helper.php)	(revision 13811)
@@ -28,7 +28,7 @@
 				}
 			}
 
-			$ret['Headers'] = implode("\n", $headers);
+			$ret['Headers'] = $headers ? implode("\n", $headers) : null; // it's null field
 
 			$lines = array_slice($lines, $line_id + 1);