There is a solution to fix this problem for Ubuntu users. If your Ubuntu has not been setup correctly and you call drupal_mail or PHP mail function, you will be getting the following error: Unable to send e-mail. Contact the site administrator if the problem persists.
To fix this problem you have to install postfix and configure it. Simply, installing postfix won't solve the problem. To install postfix refer to this stackoverflow answer To configure postfix refer to thislink. Or you can simply run
- dpkg-reconfigure postfix
and configure postfix correctly. Make sure you have not blocked 127.0.0.1 IP address that is the IP address of localhost Now you can easily send email using drupal_mail or PHP mail function.