What warm-up is, and what it is not
Every receiving filter asks the same question of a message from an unfamiliar domain: has this sender behaved like a person before? A mailbox created yesterday on a domain registered last week has no answer. Warm-up supplies one: over two to three weeks the mailbox sends and receives a rising number of ordinary conversations with seasoned mailboxes, those conversations get replies, get opened, get pulled out of spam when they land there, and get filed like real mail. By the time the mailbox writes to a stranger, the filter has seen a history that looks like a human's.
Warm-up is not a trick and it does not launder a bad reputation. If the list is scraped, the copy is spammy, open-tracking pixels are on, or the domain has already been flagged, warm-up will not save it; it will only delay the moment you find out. The vendors who sell it as a bypass are the reason providers look at warm-up traffic more carefully every year.
How the platform's warm-up works
- A network of real mailboxes, ours and our customers', exchanging varied, real-language conversations with human-like delays, never templates in lockstep.
- A ramp from the provider's floor. Each new mailbox starts at the small number its provider tolerates on day one and rises by a fixed step every day until it reaches the target, then holds. Google Workspace, Microsoft 365 and self-hosted mailboxes each have their own floor and step.
- Replies that thread. Warm-up replies quote and thread like a real conversation, which is what filters reward; a stream of unrelated one-liners is what they punish.
- Rescue from spam. When a warm-up message lands in a peer's spam folder it is marked not-spam and moved, the single strongest positive signal a receiving system records.
- Out of your way. Warm-up mail never reaches your unified inbox, never counts as a reply, and is filed out of sight in the mailbox itself.
- Maintenance after the ramp, for the life of the mailbox, so the mailbox never becomes a pure cold-sending machine with nothing coming back.
- Health that is watched. Bounces and provider refusals during warm-up pause the ramp and tell you why; a mailbox that cannot warm is a mailbox with a DNS or provider problem, and the console says which.
The ramp, in numbers
| Days | Warm-up volume | Cold campaigns |
|---|---|---|
| 1 to 3 | A handful a day, rising | None |
| 4 to 10 | Rising by a fixed step each day | None |
| 11 to 16 | At target; watching bounces, refusals and placement | None |
| 17 to 21 | At target | Start at the low end of the band: 5 a day |
| After 21 | Maintenance level for the life of the mailbox | 8 to 10 a day per mailbox; 15 is the ceiling |
Provider-specific detail, including the limits each provider publishes and the mistakes that stall a ramp, is on the Google Workspace and Outlook and Microsoft 365 pages.
What to watch while a mailbox warms
| Signal | Where you see it | What to do |
|---|---|---|
| Warm-up messages landing in peers' spam | Warm-up health per mailbox | Normal in week one; if it persists into week three, check DKIM and the domain's history |
| 5.7.x refusals from a provider | Deliveries and the mailbox's health | The platform pauses that sender for that provider; fix the cause (usually authentication or a blacklisted IP) before resuming |
| Bounces on warm-up traffic | Mailbox health | Almost always a DNS or account problem; the console names it |
| Domain reputation in Google Postmaster Tools | Postmaster Tools per domain | Should move from none to medium or high through the ramp; a fall to low means stop and investigate |
| Placement test result | Inbox placement per mailbox | Run one at the end of the ramp and before every new campaign |
What warm-up cannot fix
- A list nobody asked to be on. Verified, relevant, small. The best-warmed mailbox in the world will not survive a purchased list.
- Tracking pixels and shared link domains. Both are reliable spam signals for cold mail. The platform ships with tracking off.
- Missing or broken authentication. SPF, DKIM and DMARC must pass on every message; see the records that pass.
- Volume. Warm-up buys you 8 to 10 a day per mailbox, not 100. More volume means more mailboxes, which is an infrastructure question.
- A domain that is already burned. Replace it. Warm-up on a flagged domain is time spent proving the flag right.
Questions, answered straight
- Does email warm-up actually work?
- For what it is for, yes: it gives a brand-new mailbox and domain the sending history and positive engagement that filters expect before that mailbox writes to strangers, in two to three weeks instead of months. It does not make a bad list, tracking pixels, spammy copy or a burned domain acceptable, and no honest vendor will tell you otherwise.
- How long should I warm up a new mailbox before cold email?
- Two to three weeks from the first warm-up message to the target volume. Start campaigns at the low end of the 5 to 15 a day band in week three or four and settle at 8 to 10 a day per mailbox.
- Should warm-up keep running once campaigns start?
- Yes, at a maintenance level. A mailbox that only ever sends cold mail and receives nothing looks exactly like what it is. The platform drops warm-up to a maintenance rate when the ramp ends and keeps it there for the life of the mailbox.
- Will I see warm-up messages in my inbox?
- No. Warm-up conversations are routed so they never appear in your unified inbox or count as replies, and they are filed out of the way in the mailbox itself. Your reply rate measures real people.
- Is warm-up included in the price?
- Yes. Unlimited warm-up and unlimited mailboxes are on every plan, including the free trial.
- Can Google or Microsoft detect warm-up networks?
- They can detect anything that looks unnatural at scale: identical messages, instant replies, thousands of mailboxes talking to each other in lockstep. Ours keeps volumes low, uses varied real-language conversations with real delays, threads replies properly, and runs alongside genuine sending. Treat warm-up as the first weeks of a mailbox's normal life, not as a filter bypass.
Published 19 September 2026. Updated 19 September 2026. Written by the team that runs the infrastructure; numbers come from the platform's own provisioning and sending, and from the providers' published documentation at the time of writing.