Source

File: wp-includes/class-pop3.php

function pass ($pass = "")     {
        // Sends the PASS command, returns # of msgs in mailbox,
        // returns false (undef) on Auth failure

        if(empty($pass)) {
            $this->ERROR = "POP3 pass: " . _("No password submitted");
            return false;
        } elseif(!isset($this->FP)) {
            $this->ERROR = "POP3 pass: " . _("connection not established");
            return false;
        } else {
            $reply = $this->send_cmd("PASS $pass");
            if(!$this->is_ok($reply)) {
                $this->ERROR = "POP3 pass: " . _("Authentication failed") . " [$reply]";
                $this->quit();
                return false;
            } else {
                //  Auth successful.
                $count = $this->last("count");
                $this->COUNT = $count;
                return $count;
            }
        }
    }