The filters you can use are:
"Email Address: is empty"
"Activity was logged: subject is not empty"
This will show you all those without an email that have had a SFDC activity at all.
If you want to look at those with activities in the future, use the constraint "due date: in future".
I am trying to get a list of leads that have had no activity in SFDC. I tried using the Not Activity Logged but it pulled in contacts that still have activity.
So if you're looking for leads without an activity, pull in "Not Activity Was Logged: Subject is not empty". Clear the default constraint of "Date of Activity".
Also make sure you're using "ALL filters" in the smart list logic.