Many of Drupal's features, including user registration, depend on the server's ability to send email. Your web server needs to have a mail server available for these functions to work. Drupal uses PHP's mail() function to send e-mail (see http://php.net/mail). These two php.ini directives are needed to support mail:
SMTP = localhost smtp_port = 25
Fortunately, this is par for the course for professional LAMP (Linux, Apache, MySQL, and PHP) hosting services, and you will rarely need to worry about this when installing Drupal.
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