Authentication
Deliverability starts with authentication. If providers can't verify you're allowed to send as your domain, your mail will land in spam or be rejected.
Minimum bar:
- SPF authorizes SendLib (or your sending IPs) to send mail for your domain.
- DKIM signs the message so providers can verify it wasn't modified in transit.
- DMARC tells providers what to do when SPF/DKIM fail, and enables reporting.
Practical approach:
- Verify your sending domain early.
- Start with SPF + DKIM.
- Add DMARC with
p=noneto collect reports. - Once stable, move to
quarantine/reject.
Align domains
Keep your From domain aligned with DKIM signing and return-path handling. Misalignment is a common cause of spam placement.
Suppressions
Suppressions protect your reputation by preventing delivery to known-bad destinations.
You should automatically suppress recipients when you see:
- Hard bounces (mailbox doesn't exist)
- Spam complaints
- Unsubscribe signals (if you manage lists)
Re-sending to known-bad recipients is a reputation tax: it increases bounce rates and trains providers to distrust you.
Treat complaint signals as urgent
Complaint events are one of the strongest negative signals mailbox providers use. If complaints spike, pause marketing traffic and investigate.
Pacing and warmup
New domains and IPs don't have reputation. If you send a big blast on day one, providers will throttle or block you.
Warm up deliberately:
- Start with your most engaged recipients.
- Increase volume gradually.
- Keep content consistent while warming.
- Monitor deferrals, bounces, and complaints.
If you see throttling (421 4.7.0-style deferrals), slow down and spread traffic over time.
Warmup Calculator
Generate a simple geometric ramp. Tune it with your real-world signals.
| Day | Volume | Progress |
|---|---|---|
| 1 | 200 | |
| 2 | 285 | |
| 3 | 406 | |
| 4 | 579 | |
| 5 | 825 | |
| 6 | 1,176 | |
| 7 | 1,675 | |
| 8 | 2,388 | |
| 9 | 3,403 | |
| 10 | 4,849 | |
| 11 | 6,910 | |
| 12 | 9,848 | |
| 13 | 14,034 | |
| 14 | 20,000 |
Note: warmup is not just volume. Monitor bounces/deferrals/complaints and slow down when providers push back.
A quick deliverability checklist
- Domain verified (SPF + DKIM)
- DMARC added (start with
p=none) - Bounces and complaints wired to suppressions
- Webhooks ingested reliably (with retries + dedup)
- Volume ramp plan (pacing/warmup)
- Clear separation between transactional vs marketing traffic
Next
- Verify domain: Sending domains
- Ingest outcomes: Webhooks
- Manage suppressions: Suppressions
- Ramp safely: Warmup & pacing