wp-includes/PHPMailer/SMTP.php:1613Get the queue/transaction ID of the last SMTP transaction If no reply has been received yet, it will return null.
bool|string|null public function getLastTransactionID() { return $this->last_smtp_transaction_id; }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.