Domain reputation isn't declared, it's earned. Inbox providers track sending history per domain and IP over time, and there is no configuration setting that substitutes for that history. Warming is the process of building it deliberately, on a schedule, instead of hoping volume works itself out.
Why reputation has to be built, not declared
A domain with zero sending history looks identical, structurally, to a domain about to be used for abuse. The only signal that separates the two is behavior over time: consistent volume, low bounce rates, and real engagement in the form of opens and replies. Warming compresses the safest version of that behavior into a fixed schedule.
Warm-up first, campaigns after
Plan for a minimum of 2 to 3 weeks of warm-up before a new mailbox sends a single campaign. Longer is better if your timeline allows it. During that period the mailbox only exchanges mail with the warm-up network. Once campaigns start, ramp volume in steps rather than jumping to full capacity.
StageWeeks 1-3: warm-up
- Daily campaign sends / mailbox
- 0 (warm-up network only)
- What to watch
- Consistent delivery, no bounces, healthy Email Insights
StageFirst campaign week
- Daily campaign sends / mailbox
- 15-30
- What to watch
- Bounce rate, replies, an Inbox Placement test
StageSecond campaign week
- Daily campaign sends / mailbox
- 40-80
- What to watch
- Spam complaints, reply rate trend
StageSteady state
- Daily campaign sends / mailbox
- 100-150
- What to watch
- Week-over-week stability, InfraGuard domain score
| Stage | Daily campaign sends / mailbox | What to watch |
|---|---|---|
| Weeks 1-3: warm-up | 0 (warm-up network only) | Consistent delivery, no bounces, healthy Email Insights |
| First campaign week | 15-30 | Bounce rate, replies, an Inbox Placement test |
| Second campaign week | 40-80 | Spam complaints, reply rate trend |
| Steady state | 100-150 | Week-over-week stability, InfraGuard domain score |
Metrics to watch daily, not weekly
- Bounce rate: both the raw percentage and the split between soft and hard bounces.
- Spam complaint rate: even a handful of complaints on low volume is a meaningful signal in week one or two.
- Open rate relative to the list's historical baseline, where you have one.
- Reply rate: real engagement is the strongest positive signal a warming domain can generate.
- Blacklist status, checked at least once a day during warm-up and the first campaign weeks.
When to pull back
Specific, not vibes-based: pull back if bounce rate exceeds roughly 3 to 5% on any single day, if spam complaints exceed roughly 0.1% of sends, or if a placement test shows mail landing outside the primary inbox on more than one major provider at once. Any one of these on its own warrants action; two together warrant stopping outbound from that domain until the cause is identified.
The pull-back sequence
Halve daily volume immediately, hold for 48 hours, re-run an Inbox Placement test, and only resume the ramp from the step you were on (not from the beginning) once metrics recover.
Automating warm-up with a warm-up network
Manually warming a mailbox by having colleagues email each other back and forth doesn't scale past a handful of mailboxes. On Infrabox, each mailbox communicates with our warm-up network, where emails are exchanged automatically to simulate genuine email activity and build sender reputation. It costs $3 per mailbox per month, works for mailboxes bought through Infrabox and for existing mailboxes you connect, and keeps real prospects away from a mailbox that hasn't proven itself yet.
What happens after warm-up
Reaching steady-state volume doesn't mean monitoring stops. It means the cadence relaxes from daily to weekly, as long as metrics stay stable for two consecutive weeks. A domain that goes quiet for more than roughly two weeks (a paused campaign, a mailbox on hold) should be treated as needing a partial re-warm, not resumed at full volume on day one back.