The denominator depends on the mailbox provider or reporting system. Yahoo specifies inbox-delivered messages for its measure; that is narrower than all messages sent. (Yahoo sender best practices ↗ · Yahoo Sender Hub FAQs)
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Recomputing with the stated denominator
Illustrative Yahoo measure for May 1–31: 20 complaints ÷ 100,000 messages delivered to inbox × 100 = 0.02% for that window. Do not compare it with sent mail or another time window; Yahoo’s denominator is not a universal provider definition.
Why provider rates can differ
Using 200,000 sent messages for the same 20 complaints would produce 0.01%, half the inbox-based rate in the example. Nothing improved: only the denominator changed. Keep the provider definition and time window attached to the number.
Avoid comparing percentages built from different populations
Record the actual complaints and the denominator alongside the rate. A change in inbox delivery can change the rate even if the number of complaints is flat. When comparing two acquisition sources, use equivalent delivery measures and follow-up periods before deciding which source needs attention. Keep the denominator definition with the export, especially when sharing it across teams.