Reputation is tracked per receiving mail operator (Microsoft 365, Google
Workspace, Proofpoint, etc.), not per recipient domain -- so warmup can safely
send far more total volume if it's spread across many MX operators and throttled
per-operator. The verifier now classifies each domain's (already-cached) MX into
a provider label and writes an mx_provider column, so the warmup importer can
cap sends per operator per day. NPPES institutional sample distribution:
Microsoft 33%, Google 11%, Proofpoint ~16%, long tail across dozens of others.
