What is the smtp server Codeigniter uses?

PHP's mail function doesn't require SMTP, I think you might have a misconfiguration happening.  If you're using CI's mail to try and use PHP's mail, make sure your config is set to use PHP's mail.  See the link @ragingTouch posted.

specifically: 

PHP Code:

$config['protocol'] = 'mail';  // not SMTP or Sendmail 


Here's PHP's doc on mail.
http://php.net/manual/en/function.mail.php