WHMCS has what we think it's a bug.

If one of your clients decides to not renew a domain you can see it in the domain page, having this checked:

Even if the client expressed he doesn't want to renew the domain, WHMCS will still send the reminder email, which we think isn't useful.

We created this free hook to avoid sending emails if the domain has been disabled for auto renew.

Simply download it and upload in includes/hooks folder of your whmcs installation. You don't have to do anything else.

When The hook stops an email from being sent it creates a log in the user log (and the activity log as well) with this text:

Hook whmcstricks_do_not_send_reminder_email_for_cancelled_domains prevented email to be sent for domain domain.com

This way you can make a search and check when it took effect.