Often at times, if you do not configure the email address for CSF/LFD notifications, it will cause server's EXIM queue to be filled up, as by default the notification will be sent to root, which the root user does not accept any local email deliveries.
1. Open up /etc/csf/csf.conf with your favorite text editor
2. Look for the field: "LF_ALERT_TO", it should looks like below at a default state:
LF_ALERT_TO = ""
3. Add in your email address, it should looks like below after your modification
LF_ALERT_TO = "firstname.lastname@example.org"
4. Restart CSF
And that's it! You will now receive emails to your configured email address for your CSF/LFD notifications.