# Email warmup: why 21 days, and what real warmup looks like A new inbox has no history, and mail providers trust history. Warmup gives each inbox an ordinary working life before it writes to strangers. Here is how it works, why it can't be rushed, and how we do it. Canonical: https://slsbmb.com/guides/email-warmup/ · Updated 2026-10-30 · SLSBMB AI ## Why a new inbox needs warmup Mail providers are used to an ordinary office worker: someone who starts around nine, writes a couple of dozen unremarkable business emails a day, gets replies and keeps threads going. A brand-new inbox that starts writing to strangers on its first day looks like nothing of the kind, and within a couple of weeks its mail lands in spam. Warmup gives the inbox that ordinary history first. It is a two-sided job: to the mail provider the inbox must look like a normal office, and to the person reading the campaign email later, the sender must look like a strong professional. The limit is trust, not the technical ceiling. Google Workspace lets one user send up to 2,000 emails a day, and Google publishes no safe number for cold email: it judges a sender by authentication, reputation, complaints and how smoothly volume grows. An inbox that behaves like a mailing machine loses that trust quickly. ## How long it takes - **About 21 days.** Every SLSBMB AI inbox warms for about three weeks from the moment it exists. The first campaign emails go out on day 22. - **Then a gradual ramp.** On day 22 each inbox sends 2 campaign emails, then 2 more each day until it reaches its cap of 15. - **Counted by delivery, not by the calendar.** Progress counts only real days of warmup mail that was delivered, and the state of the system. The age of an account on its own proves nothing. - **No shortcut.** Money can't buy this history and no trick fakes it. When a client grows a hundredfold, every new inbox goes through the same path beside the old ones and inherits none of their history. - **It never stops.** Warmup continues alongside the campaigns, so an inbox keeps its living rhythm for years. ## What real warmup looks like Our AI writes the warmup mail: ordinary business conversations with their own topics and natural length. Inboxes reply to each other on a floating share of delivered emails, continue the thread in context and even arrange meetings, online and in person. No template repeats. Emails go out at an uneven, human rhythm, never exactly every few minutes, because a person can't press a button on a timer and mail providers notice when something does. We also keep control addresses at the major mail providers. They show which folder our mail actually lands in, and only observed delivery and folder count as progress; a confident "sent" counts for nothing. Where those control addresses sit and how project inboxes rotate when writing to them turned out to matter enormously. Every change to how we warm and deliver came out of experiments, more than 500 of them. Three findings were stronger than we expected: inboxes that reply to each other in context calm Google noticeably, unanswered or templated threads perform dramatically worse, and an uneven rhythm beats a perfect schedule. ## What to avoid - **Template warmup networks.** In a shared pool, your inbox swaps scripted, near-identical emails with strangers' inboxes, which open them automatically and pull them out of spam. Some of those strangers sell junk, and mail providers can link your inbox to them. - **"Pre-warmed" inboxes and domains.** You pay extra for a label, not for a history you can see. A domain's past, such as earlier mass mailings, follows it for months. - **Cheap inboxes on shared IP addresses.** If mail from your neighbours on the same address got blocked before, you start under suspicion whatever you write. - **Pushing volume per inbox.** Fifty cold emails a day from one inbox is a fast way to kill it. Grow with more domains and inboxes instead: at SLSBMB that is 8 inboxes and 120 campaign emails a day at $800 a month, and 400 inboxes and 6,000 a day at $10,000. ## The infrastructure under the warmup Warmup only works on infrastructure that is honest to begin with: - **Separate domains** that resemble your brand and lead to your website; we never send cold email from your company domain. - **Authentication**: SPF, DKIM, DMARC and MTA-STS, with matching server names and reverse DNS for every sending address. - **Our own servers and IP addresses**: four inboxes per domain, at most 60 inboxes per physical IP, each with a fixed identity, spread the way real companies are, never hundreds crowded on one address. Large mail providers keep tightening their rules. Google's [email sender guidelines](https://support.google.com/a/answer/81126) require bulk senders to use SPF, DKIM and DMARC, support one-click unsubscribe in marketing email, and keep the spam rate below 0.3%, with 0.1% as the target. Microsoft introduced similar authentication requirements for high-volume senders to Outlook.com in May 2025. Cheap infrastructure that nobody really configures doesn't survive these rules. ## Frequently asked questions ### How long should I warm up a new inbox before sending cold email? About three weeks. We warm every inbox for about 21 days, send the first 2 campaign emails on day 22 and add 2 a day until the inbox reaches 15. ### Can I skip warmup with pre-warmed inboxes? No. A pre-warmed inbox comes with a history you can't see, and the domain's past can hurt you for months. We don't connect inboxes we didn't build and warm ourselves: one bad history could harm the shared infrastructure every client sends from. ### Does warmup stop once campaigns start? No. Warmup continues alongside the campaigns for the life of the inbox, so it keeps an ordinary rhythm of conversations as well as its cold email. ### How many cold emails can a warmed inbox send per day? We cap every inbox at 15 campaign emails per sending day and grow volume with more domains and inboxes. See the full [deliverability rules](https://slsbmb.com/guides/cold-email-deliverability/) and the [capacity at each budget](https://slsbmb.com/pricing/). ## More guides - [How to choose an outbound provider](https://slsbmb.com/guides/choosing-an-outbound-provider/) - [What cold email costs in 2026](https://slsbmb.com/guides/cold-email-cost/) - [Cold email deliverability without risking your domain](https://slsbmb.com/guides/cold-email-deliverability/) - [How to calculate cold email ROI](https://slsbmb.com/guides/cold-email-roi/) - [How a cold email sequence works](https://slsbmb.com/guides/cold-email-sequence/) - [Cold email vs LinkedIn, ads, events and calls](https://slsbmb.com/guides/cold-email-vs-linkedin/) - [The first 90 days, step by step](https://slsbmb.com/guides/first-90-days/) - [How to get your first B2B customers with outbound](https://slsbmb.com/guides/first-b2b-customers/) - [Is cold email legal?](https://slsbmb.com/guides/is-cold-email-legal/) - [What to do when a prospect replies](https://slsbmb.com/guides/replying-to-prospects/) - [How to choose who to target with cold email](https://slsbmb.com/guides/who-to-target/) --- Start with a free interview: https://slsbmb.com/#start · Pricing: https://slsbmb.com/pricing/ · Contact: a@slsbmb.com