Source

File: wp-includes/class-pop3.php

function send_cmd ( $cmd = "" )
    {
        //  Sends a user defined command string to the
        //  POP server and returns the results. Useful for
        //  non-compliant or custom POP servers.
        //  Do NOT includ the \r\n as part of your command
        //  string - it will be appended automatically.

        //  The return value is a standard fgets() call, which
        //  will read up to $this->BUFFER bytes of data, until it
        //  encounters a new line, or EOF, whichever happens first.

        //  This method works best if $cmd responds with only
        //  one line of data.

        if(!isset($this->FP))
        {
            $this->ERROR = "POP3 send_cmd: " . _("No connection to server");
            return false;
        }

        if(empty($cmd))
        {
            $this->ERROR = "POP3 send_cmd: " . _("Empty command string");
            return "";
        }

        $fp = $this->FP;
        $buffer = $this->BUFFER;
        $this->update_timer();
        fwrite($fp,"$cmd\r\n");
        $reply = fgets($fp,$buffer);
        $reply = $this->strip_clf($reply);
        if($this->DEBUG) { @error_log("POP3 SEND [$cmd] GOT [$reply]",0); }
        return $reply;
    }