Source

File: wp-includes/class-pop3.php

function user ($user = "") {
        // Sends the USER command, returns true or false

        if( empty($user) ) {
            $this->ERROR = "POP3 user: " . _("no login ID submitted");
            return false;
        } elseif(!isset($this->FP)) {
            $this->ERROR = "POP3 user: " . _("connection not established");
            return false;
        } else {
            $reply = $this->send_cmd("USER $user");
            if(!$this->is_ok($reply)) {
                $this->ERROR = "POP3 user: " . _("Error ") . "[$reply]";
                return false;
            } else
                return true;
        }
    }