WP User Manager relies on WordPress wp_mail() php function to send all emails managed by the plugin. WordPress then, relies on your host to send emails. It is possible that your host is not properly configured to handle the PHP mail() function, therefore WordPress fails to send emails.
Luckily there are many tutorials and alternative ways to send emails through WordPress. Many websites integrate third party services to send emails. One of these services is called
Mandrill. Mandrill provides up to 12,000 free email deliveries a month and it’s much more reliable than the PHP mail() function.
We suggest you read the following tutorials and articles in order to understand how to setup Mandrill on your WordPress website