Scope is settled in advance so the size is never a surprise. Where a rule rejects a row, the reason travels with the rejection. Columns without a written meaning are left out rather than shipped undocumented.
Authentication
Sender records configured and verified against what the receiving side actually reports.
Contact precedence
Choosing which person represents an organisation when several qualify, by a stated rule.
Specification drafting
Writing down the field list, definitions and rules, so the same build can be repeated later.
Tell us what you tried already so we do not repeat it.