Whether your emails land in the inbox or the spam folder - the gate every other email metric sits behind, because a brilliant flow nobody receives generates nothing. Since 2024, Gmail and Yahoo enforce bulk-sender rules and non-compliant senders get rejected outright: SPF, DKIM and DMARC authentication on your sending domain, a working one-click unsubscribe, and a spam complaint rate under the line. The mistake is treating list size as an asset regardless of engagement - sending to dead contacts generates the complaints and spam-trap hits that poison inbox placement for your engaged subscribers too. A tight, engaged list is the single biggest deliverability lever.
Benchmark. Keep spam complaints under 0.10% - Gmail's hard line is 0.30%, but above 0.10% you are taking damage you cannot see until open rates slide. Authenticate every send, run the sunset flow religiously, and watch Google Postmaster Tools.