WordPress uses the default mail() function in the server. But in some of the servers, to stop spam intrusion and to add some security features this mail() function is disabled and a mailer function is used instead. If things are not configured properly, the automatic emailing system does not work which may affect some features in wordpress like the user registration process.
WordPress uses the wp_mail() function which is defined in wp-includes/pluggable.php and is dependent on the files class-phpmailer.php and class-smtp.php. If you have such problem you can try the following:
1. Download this zip file and extract the files (pluggable.php, class-phpmailer.php, class-smtp.php).
2. Edit the class-phpmailer.php. Assign values to the varibles $FromName, $Sender, $ConfirmReadingTo, $Hostname, $Host, $Port, $Username, $Password. To configure smtp for and in your server create a email id like email@yourdomain.com. This value is to be used in the variables $From, $Sender, $Username. Use its password in $Password. The usual value of $Host and $Post are mail.yourdomain.com and 25 respectively. Save the file.
3. Upload these three files to your wordpress includes directory i.e., wp-includes and replace the existing ones.
This has worked fine for me. Try for yourself.