We ask what the audience looks like before asking how many there are. Contradictions between fields are surfaced instead of being averaged away. A smaller file with known edges is preferred to a larger one without.
Manual review queue
Setting aside the cases a rule cannot settle, for somebody to look at.
Repeat control
Removing duplicates within the file and against anything previously supplied to you.
Authentication
Sender records configured and verified against what the receiving side actually reports.
If nothing suitable exists, you will hear that first rather than last.