From 54b2ab00d3d141d1418c8892a9f76dec38e1773f Mon Sep 17 00:00:00 2001
From: Marcus Bointon <marcus@synchromedia.co.uk>
Date: Mon, 6 Oct 2025 09:21:07 +0200
Subject: [PATCH] Fix $cid assembly

---
 src/PHPMailer.php | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/PHPMailer.php b/src/PHPMailer.php
index 8df4c394..242c6b38 100644
--- a/src/PHPMailer.php
+++ b/src/PHPMailer.php
@@ -4604,7 +4604,7 @@ class PHPMailer
      */
     public function msgHTML($message, $basedir = '', $advanced = false)
     {
-        $cid_domain = '@phpmailer.0';
+        $cid_domain = 'phpmailer.0';
         if (filter_var($this->From, FILTER_VALIDATE_EMAIL)) {
             //prepend with a character to create valid RFC822 string in order to validate
             $cid_domain = substr($this->From, strrpos($this->From, '@') + 1);
@@ -4631,7 +4631,7 @@ class PHPMailer
                     }
                     //Hash the decoded data, not the URL, so that the same data-URI image used in multiple places
                     //will only be embedded once, even if it used a different encoding
-                    $cid = substr(hash('sha256', $data), 0, 32) . $cid_domain; //RFC2392 S 2
+                    $cid = substr(hash('sha256', $data), 0, 32) . '@' . $cid_domain; //RFC2392 S 2
 
                     if (!$this->cidExists($cid)) {
                         $this->addStringEmbeddedImage(
@@ -4665,7 +4665,7 @@ class PHPMailer
                         $directory = '';
                     }
                     //RFC2392 S 2
-                    $cid = substr(hash('sha256', $url), 0, 32) . $cid_domain;
+                    $cid = substr(hash('sha256', $url), 0, 32) . '@' . $cid_domain;
                     if (strlen($basedir) > 1 && '/' !== substr($basedir, -1)) {
                         $basedir .= '/';
                     }