diff --git a/system/Email/Email.php b/system/Email/Email.php
index 3e4cd5671a97..6db52f3fa656 100644
--- a/system/Email/Email.php
+++ b/system/Email/Email.php
@@ -1308,7 +1308,7 @@ protected function appendAttachments(&$body, $boundary, $multipart = null)
                 . 'Content-Type: ' . $attachment['type'] . '; name="' . $name . '"' . $this->newline
                 . 'Content-Disposition: ' . $attachment['disposition'] . ';' . $this->newline
                 . 'Content-Transfer-Encoding: base64' . $this->newline
-                . ($attachment['cid'] === '' ? '' : 'Content-ID: <' . $attachment['cid'] . '>' . $this->newline)
+                . (isset($attachment['cid']) && $attachment['cid'] !== '' ? 'Content-ID: <' . $attachment['cid'] . '>' . $this->newline : '')
                 . $this->newline
                 . $attachment['content'] . $this->newline;
         }
diff --git a/tests/system/Email/EmailTest.php b/tests/system/Email/EmailTest.php
index 8d865b58ddeb..ca42613505e4 100644
--- a/tests/system/Email/EmailTest.php
+++ b/tests/system/Email/EmailTest.php
@@ -21,6 +21,7 @@
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\Attributes\Group;
 use ReflectionException;
+use ReflectionMethod;
 
 /**
  * @internal
@@ -220,6 +221,41 @@ public function testSetAttachmentCIDBufferString(): void
         );
     }
 
+    /**
+     * @see https://github.com/codeigniter4/CodeIgniter4/issues/9644
+     *
+     * @throws ReflectionException
+     */
+    public function testAppendAttachmentsWithoutCID(): void
+    {
+        $email = $this->createMockEmail();
+
+        // Manually inject an attachment without 'cid'
+        $this->setPrivateProperty($email, 'attachments', [
+            [
+                'multipart'   => 'mixed',
+                'content'     => 'VGhpcyBpcyBhIHRlc3QgZmlsZSBjb250ZW50Lg==', // base64 for "This is a test file content."
+                'filename'    => '',
+                'type'        => 'application/pdf',
+                'name'        => ['testfile.pdf'],
+                'disposition' => 'attachment',
+            ],
+        ]);
+
+        $body     = '';
+        $boundary = 'test-boundary';
+
+        // Use ReflectionMethod to call protected method with pass-by-reference
+        $refMethod = new ReflectionMethod($email, 'appendAttachments');
+        $refMethod->invokeArgs($email, [&$body, $boundary, 'mixed']);
+
+        // Assertion: Should not include a Content-ID header
+        $this->assertStringContainsString('Content-Type: application/pdf; name="testfile.pdf"', $body);
+        $this->assertStringContainsString('Content-Disposition: attachment;', $body);
+        $this->assertStringNotContainsString('Content-ID:', $body);
+        $this->assertStringContainsString('--' . $boundary . '--', $body);
+    }
+
     /**
      * @throws ReflectionException
      */
diff --git a/user_guide_src/source/changelogs/v4.6.3.rst b/user_guide_src/source/changelogs/v4.6.3.rst
index 6b44883af986..fa7786891956 100644
--- a/user_guide_src/source/changelogs/v4.6.3.rst
+++ b/user_guide_src/source/changelogs/v4.6.3.rst
@@ -30,6 +30,8 @@ Deprecations
 Bugs Fixed
 **********
 
+- **Email:** Fixed a bug with CID check when building email attachments.
+
 See the repo's
 `CHANGELOG.md <https://github.com/codeigniter4/CodeIgniter4/blob/develop/CHANGELOG.md>`_
 for a complete list of bugs fixed.