The five things that drive inbox placement, ranked by impact:
1. Domain authentication. DKIM + SPF + DMARC must all align. If any one fails, you bleed reputation. Use the dashboard Domains tab to verify.
2. Complaint rate. Stay under 0.1%. Above 0.3% and you will see degradation across Gmail and Outlook within days.
3. Bounce rate. Stay under 2%. Above 5% triggers temporary blocks at major providers. Pre-send validation is the leverage point — it drops bounces by 90%+.
4. Engagement. Open and click rates above 15% trend positive. Below 5% trends negative. Suppress inactive subscribers proactively.
5. Volume consistency. Sudden 10x spikes trigger anti-spam heuristics. Either warm up properly or stay within 2x of your rolling average.