---
title: Email warmup
description: Why warmup exists, how providers read it, and how to do it without faking signals.
---

Warmup is the practice of sending small, conversational volumes of mail from a new (or recently dormant) mailbox to gradually build a positive reputation with mailbox providers like Google, Microsoft, Yahoo and Apple. The goal isn't a one-off score. It's a track record. Mailbox providers reward consistency.

## What "warmup" actually means

Modern warmup tools generate this traffic between participating mailboxes: your mailbox sends short, plain-text messages to other warmed mailboxes, those mailboxes open and reply, and the cycle repeats. Done well, the pattern looks like a real human starting to use a new inbox.

## Why providers care about it

Inbox placement is reputation-driven. When a brand-new domain or mailbox starts sending dozens of cold messages to strangers, Gmail and Outlook treat it as suspicious by default. They don't know whether you're a small startup or a spammer, so the safest assumption, for them, is spammer.

Warmup gives them signal. Mailboxes that send small volumes, get opens, get replies, and don't trigger complaints look like real human inboxes. Providers loosen the screws as that pattern persists.

## What good warmup looks like

### Gradual volume growth

Start low and ramp slowly. Warmbly's defaults are 10 emails per day to start, +1 per day, with a ceiling of 40 per day. That ceiling exists on purpose: warmup that climbs to 100/day looks unnatural and starts to look like a bot.

### Realistic spacing

Warmup mail should be spread across the working day, with sensible gaps. Sending all 40 messages in a single minute is the loudest possible spam signal.

### Conversational behaviour

A real mailbox doesn't only send. It replies. Warmup pools should generate a mix of new sends and replies to existing threads. Warmbly defaults to a 30% reply rate within the pool, tunable per mailbox.

### Quality recipients

Warmup recipients should be other warmed, healthy mailboxes: not free trial mailboxes, not throwaways, not lookalikes of catch-all domains. Pool quality matters more than pool size.

### Token verification

Every warmup message Warmbly sends carries a verification token. Missing, malformed or replayed tokens are tracked per mailbox and feed a rolling spam score. Above the threshold, the mailbox is removed from the pool until it requalifies.

A token carried in a header only works if the header survives delivery, and not every provider passes custom headers through. Warmbly records the subject and the provider-assigned message identifier for each send as well, so a warmup email whose header was stripped in transit is still recognized at the recipient rather than being treated as ordinary mail.

## What bad warmup looks like

- **Synthetic blasting**: sending 200 identical messages a day to the same handful of recipients. Looks great on a vendor dashboard. Looks awful to Gmail.
- **Throwaway pools**: generating recipient mailboxes from free providers on the fly. Providers know what disposable inboxes look like and weight signals from them at zero, or worse.
- **Open/click theatre**: simulating opens and clicks at unrealistic rates. Apple Mail Privacy already pollutes open rates; pumping the number from a bot makes the entire signal worthless.
- **Stopping at "ready"**: many tools stop warmup the moment campaigns begin. Reputation has to be maintained, not earned once and forgotten.

## How long does it take?

Domain warmup commonly takes 3–6 weeks to reach a stable state. New domains take longer than re-warming an existing one. New mailboxes on warmed domains often stabilise within a week or two if the domain reputation is already healthy.

What matters is not _finishing_ warmup: it's running it indefinitely at a low volume in parallel with your cold sending, so any reputation dip is caught and offset before campaigns hit it.

## Free pools vs paid pools

Free warmup pools are useful for testing but mix higher-risk participants. Paid pools should be stricter: only vetted, monitored mailboxes participate, and misbehaving senders are quarantined fast.

In Warmbly, free-tier mailboxes never silently appear in the premium pool. Even dedicated-worker customers participate in the premium pool by default. Dedicated infrastructure isn't a reputation shortcut, it's an isolation tool.

## When to quarantine a mailbox

Most platforms wait until a mailbox is landing in spam 80% of the time before they pull it. By then the reputation damage is already done. Acting earlier protects everyone in the shared pool. Our policy:

- **Watch**: spam placement ≥ 10% over a 20-delivery sample. Lower volume, increase spacing.
- **Quarantine**: spam placement ≥ 20%, or complaint rate ≥ 0.10%, or bounce rate ≥ 5%. 7-day removal from the paid pool.
- **Block**: spam placement ≥ 40%, or complaint rate ≥ 0.30%. 30-day block, manual review to re-enter.
- **Catastrophic**: spam placement ≥ 80%. Long block plus reputation reset, with no automatic re-entry.

## Practical checklist

1. Set up SPF, DKIM and DMARC before you start warming.
2. Choose plain text. Save HTML for transactional mail.
3. Use a sub-domain for cold sending (e.g. `outreach.acme.com`) so any reputation dip is isolated from your transactional domain.
4. Start at 10/day, ramp +1/day, ceiling 40/day.
5. Reply rate inside the pool around 30%.
6. Don't stop warmup when you start campaigns.
7. Watch placement weekly, not opens daily.

> Warmup is reputation insurance, not a magic switch. Treat it like a habit your mailbox maintains, not a project you finish.
