Fatal error: Cannot instantiate non-existent class: simplemail

Code:

<?php

class SimpleMail {
  public $to = NULL;
  public $cc = NULL;
  public $bcc = NULL;
  public $from = NULL;
  public $subject = '';
  public $body = '';
  public $htmlbody = '';
  public $send_text = TRUE;
  public $send_html = FALSE;
  private $message = '';
  private $headers = '';
  
  public function send($to = NULL, 
                       $subject = NULL, 
                       $message = NULL, 
                       $headers = NULL) {
    if (func_num_args() >= 3) {
      $this->to = $to;
      $this->subject = $subject;
      $this->message = $message;
      if ($headers) {
        $this->headers = $headers;
      }
      
    } else {
    
      if ($this->from) {
        $this->headers .= "From: " . $this->from . "\r\n";
      }
      if ($this->cc) {
        $this->headers .= "Cc: " . $this->cc . "\r\n";
      }
      if ($this->bcc) {
        $this->headers .= "Bcc: " . $this->bcc . "\r\n";
      }

      if ($this->send_text and !$this->send_html) {
        $this->message = $this->body;
      } elseif ($this->send_html and !$this->send_text) {
        $this->message = $this->htmlbody;
        $this->headers .= "MIME-Version: 1.0\r\n";
        $this->headers .= "Content-type: text/html; " .
                          "charset=iso-8859-1\r\n";
      } else {
        $_boundary = "==MP_Bound_xyccr948x==";

        $this->headers = "MIME-Version: 1.0\r\n";
        $this->headers .= "Content-type: multipart/alternative; " .
                          "boundary=\"$_boundary\"\r\n";
        
        $this->message = "This is a Multipart Message in " . 
                         "MIME format\n";
        $this->message .= "--$_boundary\n";
        $this->message .= "Content-Type: text/plain; " .
                          "charset=\"iso-8859-1\"\n";
        $this->message .= "Content-Transfer-Encoding: 7bit\n\n";
        $this->message .= $this->body . "\n";
        $this->message .= "--$_boundary\n";
        $this->message .= "Content-type: text/html; " . 
                          "charset=\"iso-8859-1\"\n";
        $this->message .= "Content-Transfer-Encoding: 7bit\n\n";
        $this->message .= $this->htmlbody . "\n";
        $this->message .= "--$_boundary--";     
      }
    }

    if (!mail($this->to,$this->subject,$this->message,$this->headers)) {
      throw new Exception('Sending mail failed.');
      return FALSE;
    } else {
      return TRUE;  
    }
  }

}

?>