diff --git a/Mail.class.php b/Mail.class.php
index bdc6680..64f264b 100644
--- a/Mail.class.php
+++ b/Mail.class.php
@@ -22,7 +22,7 @@ class Mail {
     }
 
     function add_attachment($content, $filename=null, $cid=null) {
-        $this->mailer->addStringEmbeddedImage($content, $cid, $filename);
+        $this->mailer->addStringEmbeddedImage($content, $cid ? : bin2hex(random_bytes(10)), $filename);
     }
 
     function add_attachments($attachments) {