What is an IP warming plan and what metrics should you monitor during the warm-up period?

IP warming is the process of gradually increasing sending volume from a new or cold IP address to build a positive sending reputation with ISPs before sending full volume.
Typical ramp-up: Start with 500–1,000 emails/day to your most engaged subscribers (opened in last 30 days). Double volume roughly every 2–3 days, targeting engaged segments before the broader list.
Seed lists: Include internal seed addresses at major MBPs (Gmail, Yahoo, Outlook.com) to monitor inbox placement during warm-up.
Metrics to monitor:
— Bounce rate: should stay below 2% (hard bounces under 0.5%).
— Spam complaint rate: must stay below 0.08% (Gmail threshold); above 0.3% causes serious reputation damage.
— Open rate: target at least 20% from warmed segments.
— Spam trap hits: monitored via tools like Validity/Return Path.
— Inbox placement rate: percentage landing in inbox vs spam vs missing. Target above 90%.