This is likely a server configuration issue.
Elgg needs access to sendmail as described here in order for email to be sent. Check the email configuration in php.ini.
If this is an occasional problem, your ISP may be limiting the number of emails that you can send out in an hour and you are running up against that limit.
You need to check whether if your server has sending emails capability.
Rodolfo Hernandez
Arvixe/Elgg Community
info@elgg.org
Security issues should be reported to security@elgg.org!
©2014 the Elgg Foundation
Elgg is a registered trademark of Thematic Networks.
Cover image by Raül Utrera is used under Creative Commons license.
Icons by Flaticon and FontAwesome.