Registration problem
Unable to send email. Contact the site administrator if the problem persists.

This error is due to email sending failed.

You can use SMTP Authentication Support module for sending email.


  This module allows Drupal to bypass the PHP mail() function and send
  email directly to an SMTP server. The module supports SMTP
  authentication and can even connect to servers using SSL if supported
  by PHP.


You can take the picture as reference for SMTP configuration.