Built to a definition, not to a round number.
Work is scoped against what the sources can actually support. Verification is separated from enrichment, so neither hides the other. The delivered count and the approved count are the same number.
Sending identity
Separating what you send in volume from how your main domain is treated.
Second-opinion pass
Having somebody who did not build the file check it against the brief.
Date normalisation
Putting every date in one format and one timezone before any comparison runs.
Bring the brief that other suppliers found difficult.