mail is not send when the create new user and notify that user

I have made site using drupal-7 there is the one issue i m facing is that, When开发者_如何学运维 i m create new user i m check the checkbox notify user but the email is not send and message is come like Unable to send e-mail. Contact the site administrator if the problem persists.

i can not understand where the actual problem is

Thank in advance

Are you able to send other emails from your site? If not, and if the server where the site is cannot send email from localhost, then maybe you could try configuring an SMTP server via this module: http://drupal.org/project/smtp