I am using the $FILTER variable but an email from EventSentry shows "Unknown Filter" instead of the actual filter name.

EventSentry Version 2.60 and earlier:
This occurs only if you are using the summary notification feature and restarted the EventSentry service (or the entire computer) during the collection period.

EventSentry Version 2.70:
Starting with EventSentry 2.70 the $FILTER variable is no longer supported for summary notifications due to an internal change in the feature. It is recommended that you remove the $FILTER variable from the email subject or explicitely enter the name of the filter in the subject.