"Did it send?" is the wrong question. "Did it reach the inbox, and did people want it?" is the right one. These are the metrics that tell you, and the thresholds that should make you act.
Delivery rate vs inbox placement
Delivery rate only tells you the receiving server accepted the message — it says nothing about whether it landed in the inbox or the spam folder. Inbox placement is the number that matters. Seed tests and provider tools help you estimate it.
Bounce rate
Bounces are messages that couldn't be delivered. Hard bounces (the address doesn't exist) should be removed immediately. Keep your bounce rate under about 2% — higher tells providers your list is stale.
Complaint rate
The percentage of recipients who mark you as spam. This is the metric mailbox providers watch most closely.
Open and click rates
Engagement signals that mail is wanted. Falling opens over time suggest list fatigue — prune inactive subscribers before providers start filtering you.
Spam-trap hits
Spam traps are addresses that should never receive mail. Hitting them is a strong sign of a purchased or poorly maintained list, and it damages reputation quickly. The fix is upstream: only ever mail people who opted in.
A simple dashboard to keep
| Metric | Healthy | Act if |
|---|---|---|
| Bounce rate | < 2% | > 5% |
| Complaint rate | < 0.1% | > 0.3% |
| Inbox placement | > 95% | < 85% |
Our higher SMTP tiers include bounce and complaint reporting so these numbers are in front of you. Pair them with correct authentication and a clean list and deliverability becomes something you manage, not something that happens to you.