Boost inbox placement by aligning authentication, sending reputation, and content signals with Gmail’s 2026 spam filters. Focus on SPF/DKIM/DMARC alignment, warm IP ramp‑up, and engagement‑driven subject lines.
Step‑by‑step checklist
1. Authenticate every domain
- SPF: include all sending IPs (include:mail.example.com).
- DKIM: use RSA‑2048 keys, rotate every 90 days.
- DMARC: set p=reject; rua=mailto:postmaster@example.com; ruf=mailto:forensics@example.com; pct=100.
```dns
_dmarc.example.com. IN TXT "v=DMARC1; p=reject; rua=mailto:postmaster@example.com; ruf=mailto:forensics@example.com; pct=100"
```
2. Warm new IPs – start at 1 k messages/day, double every 48 h until reaching target volume, monitoring the Gmail Postmaster “Spam Rate” (<0.1 %).
3. Segment by engagement – calculate a 30‑day score:
```python
score = (opens0.4 + clicks0.5 + replies*0.1) / sends
```
Only send to contacts with score >= 0.25.
4. Craft subject lines – keep length 35‑50 chars, front‑load a personalized token, avoid all‑caps and excessive punctuation. Test with Gmail’s “Inbox Placement” API (available via Postmaster Tools).
5. Use the Promo‑tab bypass header – add X-AMP-HTML with a minimal AMP email to signal primary content.
6. Monitor Postmaster metrics – keep “Spam Rate” <0.1 %, “Feedback Loop” complaints <0.05 %, and “Domain Reputation” >90.
7. Validate HTML – pass the Gmail HTML validator (gmail-html-validator npm package) with a score >95 %.
Quick comparison
| Metric | Good | Bad |
|-----------------------|---------------|-------------------|
| SPF/DKIM alignment | Pass | Fail |
| DMARC policy | reject/pct=100| none/quarantine |
| Spam Rate (Postmaster)| <0.1 % | >0.5 % |
| Engagement score | ≥0.25 | <0.10 |
Gotcha: Exceeding a 20 % hard‑bounce rate triggers Gmail’s “low‑reputation” flag, overriding perfect authentication and dropping you into the Spam tab.