You'll need to be more specific. Is there an error in your error log? Do they arrive in the spam folder? Are you using the PHP mail() function (default) or SMTP?
You'll need to be more specific. Is there an error in your error log? Do they arrive in the spam folder? Are you using the PHP mail() function (default) or SMTP?
Post the full error here. If you're using SMTP, you need to check your settings. If you're using the built-in mail function, you need to check the upstream error log (at /var/log/maillog or similar) on your server.