Since the Google and Yahoo bulk-sender rules of February 2024 (Microsoft followed in May 2025), inbox placement is a hard technical gate, not a soft best practice. The 0.30% spam cap governs how much you can send at all, so I treat it as the binding volume constraint.
Shape of it
4 signals
Configuration and reputation state of sending domains and mailboxes.
Benchmark2–3 mailboxes/domain; plan on 30–50 cold sends/inbox/day (specialists say 15–25), not the 100–150 vendors claim
Infrastructure design lets you scale volume without concentrating reputation risk.
Spam complaint rate as reported by Google Postmaster Tools.
BenchmarkBelow 0.30% hard line; target <0.10%. Q4 2025 cold inbox placement ran Gmail 57%, Outlook 45%, so delivered is not inboxed
The metric mailbox providers actually enforce against you.
The 2–4 week schedule bringing a new mailbox up to sending volume.
Benchmark2–4 weeks (6–12 for full maturity), then ramp ≤20%/week
Warmup is one pillar, not a silver bullet; ramping too fast still gets a new domain flagged.
Share of sends rejected (see Outbound systems).
BenchmarkUnder 2%
The fastest way to wreck a domain; gate every send on list hygiene.