Email warm-up: the exact ramp we enforce in code

A new sending domain can safely send about 150 emails on its first day, and about 1,100 a day by the end of week one. Push volume faster than that curve and mailbox providers read the growth itself as a spam signal, because a fresh domain blasting thousands of messages is the oldest pattern in the spam playbook. Email warm-up is the discipline of ramping inside the curve. This article publishes the exact curve our send path computes, the relay ceiling it is capped by, and the reasoning behind every number.

What warm-up actually warms

Mailbox providers score two things: the sending IP and the sending domain. If you send through a relay's shared pool, the IPs carry years of history maintained by thousands of senders, and nothing you do in your first month moves them. What has no history is your domain. Warm-up is the process of writing that history on purpose: modest volume, engaged recipients, steady cadence, until providers have enough data to score you on behavior instead of novelty. New-domain reputation matures over roughly 4 to 8 weeks; the full policy this article draws on lives in our email deliverability handbook.

Warm-up cannot fix a broken foundation. Gmail has rejected unauthenticated mail outright since November 2025, and no ramp schedule recovers a message that bounced with a 550. If you are not certain your SPF, DKIM and DMARC records pass, run the six spam checks first and come back.

The ramp table, and why each number

This is the schedule our throttle enforces. It is code, not a guideline: the send path computes the day's cap and refuses to exceed it.

Why 150 and 1.4: those are Resend's documented domain-warming numbers (resend.com warm-up docs, checked June 2026), and they compound to about 1,100 a day by day 7. We adopted them rather than inventing our own because the relay observes far more sending than we do.

Why the gates: the 0.1% complaint line and the 2% bounce-rate ceiling come straight from Google's bulk-sender rules (February 2024) and standard list-hygiene practice. Our throttle checks both before allowing the next day's increase, and the send path halts entirely at 0.3% complaints or 2% bounces. No human judgment sits in that loop, deliberately.

Why most-engaged first: providers weight replies above clicks and clicks above opens. Week one is an audition, so stack the audience. Sending your first 150 emails to people who reliably open and reply writes exactly the history you want a new domain to have.

One honest note on our own scale: LetterDuck has run as a live single-tenant workspace since June 2026, and our real volumes are small. That is precisely why the early ramp costs us nothing to follow, and at our size the relay's own 100-a-day free-tier cap enforces day-one discipline before our throttle even gets a vote.

Who to send to during warm-up

Your most-engaged 90-day segment, and nobody else. Not the full list with fingers crossed, and never a cold or imported list. Warm-up weeks are when providers are actively forming an opinion; a bounce spike or a complaint in week one weighs more than the same event will weigh in month six, because it is a larger share of your total history.

If you do not have an engaged segment yet because the domain and the list are both new, warm up on genuinely wanted mail: signup confirmations, replies, receipts. Volume matters less than reaction.

Shared IP or dedicated: do the math first

Below about 50,000 emails a month, a dedicated IP is a mistake. We hold this position and here is the arithmetic: an IP needs steady daily volume to hold its reputation, Postmark puts the worthwhile threshold around 300,000 emails a month, and AWS SES guidance points to roughly 100,000 a day (both from their published docs, checked June 2026). Send 20,000 a month through a dedicated IP and it spends most of its life cooling off, and a cold dedicated IP delivers worse than a busy shared pool.

The tradeoff runs the other way too, so name it: on a shared pool you inherit your neighbors, good and bad. Part of what you pay a relay for is pool policing, which means suppression enforcement and evicting abusers. Pick a relay that visibly does this. On shared infrastructure you warm only the domain, which is the cheaper half of the job, and it is the half this article covers.

What breaks warm-up

Four things undo weeks of progress.

Gaps. Cadence is itself a signal, and Outlook in particular rewards consistent history. A two-week silence in the middle of a ramp reads as a different sender resuming, so restart below where you stopped, not at the old cap.

Bursts. Hold growth under 30 percent a day as an absolute ceiling, with 10 to 20 percent the ideal. Tripling volume overnight looks like a compromised account or a freshly bought list, because it usually is.

Cold list imports. Dumping an old list into a mid-ramp domain spikes bounces past the 2% gate in a single send. Clean and segment first, import later.

Identity churn. Changing the From name or address resets recipient recognition and the filters' model of you. Pick a From identity before day 1 and keep it.

Warm-up tools, honestly

Warm-up tools are networks of inboxes that open your mail, reply to it, and drag it out of the spam folder automatically, to simulate engagement history. Our take: for a newsletter or normal business mail, they solve the wrong problem. Gmail's filtering is personalized per recipient, so warmth manufactured inside a seed network does not transfer to the real humans on your list, and the engaged-first ramp above builds the same history out of genuine behavior. The tools have a narrower case in cold outreach, where there is no opted-in audience to ramp against. The limitation of our position, stated plainly: we run newsletters and business inboxes, not high-volume cold outreach, and our bias follows our workload. We rank the four main subscriptions by mechanism and published price at our warm-up tools roundup.

The whole discipline in four lines

Day 1 is about 150 emails to the people most likely to reply. Multiply by roughly 1.4 a day through day 7. From day 8, grow 10 to 20 percent daily, and if complaints touch 0.1% or bounces touch 2%, hold until they recover. Silence and spikes are the only two ways to fail, so avoid exactly those two things.