wp-includes/PHPMailer/SMTP.php:1181Send an SMTP NOOP command.
bool public function noop() { return $this->sendCommand('NOOP', 'NOOP', 250); }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/SMTP.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.