“Deliverability” gets discussed like weather—mysterious, ambient, unaccountable. It isn’t. Two operational metrics predict most of it, and both are on a dashboard you already have.
The two numbers
- Bounce rate: keep it under 4%. Repeatedly sending to invalid addresses tells providers you don’t know your own list—the signature move of a spammer with a scraped database. Bounces are also the earliest warning that your list is aging.
- Spam complaint rate: keep it under 0.08%. That’s the healthy zone; 0.3% is the ceiling Gmail, Yahoo, and Microsoft enforce on bulk senders before filtering gets punitive. Complaints are recipients telling the referee, not you.
One definitional trap while we’re here: delivery is not deliverability. “Delivered” means the receiving server accepted the message; it says nothing about inbox versus spam folder. A 99% delivery rate with mail rotting in spam is a deliverability failure your delivery metric will never show—which is exactly why you watch bounces and complaints instead.
How to actually watch them
- Your ESP’s dashboard. On Resend, the Metrics tab charts bounce and complaint rates continuously, and per-email Deliverability Insights flag issues before they become patterns.
- Webhooks for automation. Subscribe to
email.bouncedandemail.complainedevents and wire them to alerts—and to your list, so offenders are removed automatically. A suppression list does the removal part natively. - Gmail’s blind spot. Gmail doesn’t report individual complaints back to senders, so your ESP-side complaint rate undercounts your biggest audience. Gmail Postmaster Tools is where Gmail shows you its number—watch both.
When the numbers move
Rising bounces → clean the list and check what changed at the source (a stale segment? a broken signup form?). Rising complaints → your content or targeting outran consent: make unsubscribing effortless, tighten to engaged recipients, and slow down. Both respond fast if you respond fast—the expensive failures are the ones nobody was watching.