This problem happens in some rare cases. Usually, the problem is with your mail server (SMTP) which is not sending e-mails, this can be fixed by using WP Mail SMTP by WPForms plugin in your WordPress, it will configure your mail server and should make it work.
Note:
- This is not a theme problem, but this is a server related problem, for more informations you should contact your hosting provider.
- If you are in localhost emails will not be delivered.