diff --git a/src/wp-includes/class-smtp.php b/src/wp-includes/class-smtp.php
index 7b93997f73..524e655250 100644
--- a/src/wp-includes/class-smtp.php
+++ b/src/wp-includes/class-smtp.php
@@ -10,6 +10,6 @@ _deprecated_file(
 	__( 'The SMTP class has been moved to the wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace.' )
 );
 
-require __DIR__ . '/PHPMailer/SMTP.php';
+require_once __DIR__ . '/PHPMailer/SMTP.php';
 
 class_alias( PHPMailer\PHPMailer\SMTP::class, 'SMTP' );