How to read a warmup curve and know if a mailbox is healthy
A clean curve looks specific. A bad curve has a clear shape. Here is what each one means.
Rejwan Nirob·Jan 19, 2026·5 min readA warmup curve is a daily count of the messages a mailbox has sent during its warmup period. Plotted over 14 days, the shape tells you whether the mailbox is healthy, behind schedule, or compromised.
The healthy curve
Smooth ramp from 5 messages on day 1 to 20 messages on day 14, with daily increments of roughly 1 message per day. Open rate during warmup should be near-100% (synthetic warmup network). Reply rate should be 30 to 60%. Spam rate should be near-zero. We deliberately ramp to 20, not the platform ceiling, because sustained sends above 20/day correlate with placement decline.
The behind-schedule curve
Daily volume below the ramp target (e.g. day 7 still at 10 messages instead of 30). Usually caused by quota limits in the sending tool, not a deliverability problem. Fix the cap, the curve resumes.
The compromised curve
Volume on target but open rate dropping below 80%. Indicates the warmup network is partially landing in spam. Pause sending, investigate SPF/DKIM/DMARC drift, then resume from the same warmup day.
When to call a mailbox done
- Day 14 reached at full volume
- Open rate above 90% across the full warmup
- Spam rate near zero across the full warmup
- Seed-list placement test passes for primary inbox
The Inboxlee dashboard reads the warmup curve from your sending tool API and renders it per mailbox. Healthy curves are green. Compromised curves trigger an alert.
Frequently asked
What does a healthy cold-email warmup curve look like?
Smooth ramp from 5 messages on day 1 to 20 messages on day 14, with daily increments of roughly 1 message per day. Open rate during warmup should be near-100% (the warmup network auto-opens). Reply rate should be 30 to 60% (auto-replies). Spam rate should be near-zero. Inboxlee renders this per mailbox in green when the curve is on track.
My mailbox warmup is behind schedule - what is wrong?
Almost always a quota cap in the sending tool, not a deliverability problem. Open the mailbox in your sender, raise the daily limit to match the warmup target, and the curve resumes from the same day. Behind-schedule curves with healthy engagement metrics are a configuration issue, not a reputation problem.
My warmup open rate is dropping - what does that mean?
The warmup network is partially landing in spam. This is a compromised curve - pause sending immediately, investigate SPF/DKIM/DMARC drift (re-run dig on each record), check for new TXT records that conflict, and resume warmup from the same day once the auth gap is fixed. Do not continue sending - you are training Gmail to flag the mailbox.
When is a cold-email mailbox done warming up?
Four conditions, all required: day 14 reached at full ramp volume, open rate above 90% across the full warmup, spam rate near zero across the full warmup, and seed-list placement test passes for primary inbox. Missing any one means warm more - the mailbox is not production-ready.