wp-includes/PHPMailer/POP3.php:396Send raw data to the POP3 server.
$stringstringint protected function sendString($string) { if ($this->pop_conn) { if ($this->do_debug >= self::DEBUG_CLIENT) { //Show client messages when debug >= 2 echo 'Client -> Server: ', $string; } return fwrite($this->pop_conn, $string, strlen($string)); } return 0; }Unchanged from 6.9.7 through 7.1.0.
Signature, return type and hooks compared across 3 parsed releases.
src/wp-includes/PHPMailer/POP3.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.