You may also want to modify the rule to use a template with more information slots so you can get things like the "Detection IP" in the e-mail and have it tell you which machine is running low on disk space.
Also, that Windows event is based on percentages, not free bytes. It fires when a disk has less than 5% free. If a server has a 50GB drive, that may be a real concern. If it's a system with 64TB of disk, 5% is still a lot.
I have this video on configuring e-mails in rules that may help.