Preparation is cheaper than a damaged sending reputation. The checks happen before the first message, which is the only point they are worth anything.
Segment design
Deciding the cuts with you, against what the data can actually support.
Suppression
Exclusion lists applied properly, so a record you asked to withhold stays withheld.
Coverage by field
Which fields are populated and how often, reported per segment instead of as one average.
A short description is enough to begin. We will come back with what it implies.