UPDATE

wp-includes/class-phpmailer.php

@@ -0,0 +1,19 @@
+<?php
+
+/**
+ * The PHPMailer class has been moved to the wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace.
+ */
+if ( function_exists( '_deprecated_file' ) ) {
+	_deprecated_file(
+		basename( __FILE__ ),
+		'5.5.0',
+		WPINC . '/PHPMailer/PHPMailer.php',
+		__( 'The PHPMailer class has been moved to wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace.' )
+	);
+}
+
+require_once __DIR__ . '/PHPMailer/PHPMailer.php';
+require_once __DIR__ . '/PHPMailer/Exception.php';
+
+class_alias( PHPMailer\PHPMailer\PHPMailer::class, 'PHPMailer' );
+class_alias( PHPMailer\PHPMailer\Exception::class, 'phpmailerException' );