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, keep genuinely dead contacts off your sends (bounced, never opened, silent long past your repurchase cycle), and watch Google Postmaster Tools.