Ticket #33972: 33972-fix-wp_mail-stale-state.patch

src/wp-includes/pluggable.php

@@ -390 +390 @@
         global $phpmailer;
 
-        // (Re)create it, if it's gone missing or been destroyed
-        if ( ! ( $phpmailer instanceof PHPMailer\PHPMailer\PHPMailer ) ) {
-                require_once ABSPATH . WPINC . '/PHPMailer/PHPMailer.php';
-                require_once ABSPATH . WPINC . '/PHPMailer/SMTP.php';
-                require_once ABSPATH . WPINC . '/PHPMailer/Exception.php';
-
-                $phpmailer = new PHPMailer\PHPMailer\PHPMailer( true );
-        }
+        /*
+         * Always create a new PHPMailer instance to ensure that no residual state
+         * such as Encoding, attachments, or headers persist between wp_mail() calls.
+         * This fixes stale configuration issues introduced by static use of $phpmailer.
+         */
+
+        require_once ABSPATH . WPINC . '/PHPMailer/PHPMailer.php';
+        require_once ABSPATH . WPINC . '/PHPMailer/SMTP.php';
+        require_once ABSPATH . WPINC . '/PHPMailer/Exception.php';
+
+        $phpmailer = new PHPMailer\PHPMailer\PHPMailer( true );
+
+        // Ensure Encoding resets to default (8bit)
+        $phpmailer->Encoding = '8bit';