---
title: Warmup
description: "How email warmup builds sender reputation, pools, and ramp."
icon: Flame
---

Warmup keeps your mailboxes healthy by sending small amounts of low-risk, natural-looking mail between participating accounts. The goal is to build and maintain sender reputation gradually, so that when you start (or continue) cold outreach, your messages are more likely to land in the inbox instead of spam.

Warmup runs quietly in the background. You turn it on for a mailbox, choose how aggressively it ramps, and Warmbly handles partner selection, spacing, replies, and safety on your behalf.

<Callout type="info" title="Warmup is a paid feature">
  Warmup is available on paid plans only. Free organizations can connect mailboxes, but warmup traffic will not be scheduled for them. If your plan does not include warmup, any pending warmup activity for your mailboxes is skipped rather than sent.
</Callout>

## Why warmup matters

Mailbox providers like Google and Microsoft watch how a sending address behaves over time. A brand-new address that suddenly sends a burst of cold email looks suspicious and is likely to be filtered. An address that sends a steady, modest, conversational stream of mail and gets replies looks like a real person.

Warmup gives your mailbox that history. It:

- generates normal-looking, low-volume activity instead of sudden spikes
- spreads sends across the day with natural spacing
- mixes in replies so threads look like real conversations
- ramps volume up slowly so the increase looks organic

Reputation is built slowly and lost quickly, so warmup favors patience over throughput.

## How warmup works

When warmup is enabled on a mailbox, Warmbly repeatedly:

1. Confirms your organization is allowed to use warmup.
2. Picks a partner mailbox from your warmup pool, avoiding partners it used recently.
3. Decides whether to start a fresh message or reply to an existing warmup thread (based on your reply rate).
4. Sends a short, plaintext, unremarkable message through the worker assigned to that mailbox.
5. Records the send, updates your daily warmup stats, and schedules the next send.

Each warmup send carries a hidden verification token so the receiving side can confirm the message is genuine warmup traffic and not abuse. Warmup mail is plaintext only and is not tracked with open or click pixels, because the point is to look like ordinary personal email.

## Pools: free vs premium

Warmup traffic is separated into pools. A pool is a set of mailboxes that warm each other up. Your mailbox only ever exchanges warmup mail with other mailboxes in the same pool.

| Pool | Who is in it | Notes |
|------|--------------|-------|
| `premium` | Paid organizations' mailboxes | The default pool for paid accounts; isolated from lower-trust traffic. |
| `free` | Lower-trust or trial mailboxes | Kept separate so free traffic never mixes into premium reputation. |

This separation is deliberate and is never crossed silently. Premium pool quality is protected so that healthy paying mailboxes are not exposed to the reputation risk of unproven inboxes.

A larger pool gives each mailbox more partners to rotate through, which makes warmup look more natural. Warmbly actively spreads your sends across many partners and many recipient domains rather than looping the same two mailboxes back and forth, since tight reciprocal pairs are an easy pattern for providers to detect. Pool quality matters more than raw pool size.

## The ramp

Warmup volume increases gradually instead of jumping straight to a high number. Each mailbox follows a simple ramp you can configure:

| Setting | Default | Meaning |
|---------|---------|---------|
| Start volume | `10` per day | How many warmup emails the mailbox sends on day one. |
| Daily increase | `+1` per day | How much the daily target grows each day. |
| Ceiling | `40` per day | The maximum daily warmup volume; the ramp stops climbing here. |

So a mailbox at default settings sends about 10 warmup emails on its first day, 11 the next day, 12 the day after, and so on, leveling off at 40 per day after roughly a month. You can adjust these values per mailbox, but the conservative defaults exist because slow ramps build the most durable reputation.

A few things shape the actual number sent on a given day:

- Sends are spread across your configured warmup hours with human-like spacing and jitter, never all at once.
- The daily target is capped by how many eligible partner mailboxes exist, so a small pool will not push a mailbox to send the same partner twice in one day.
- If a mailbox's health drops (see below), its volume is automatically reduced and its spacing widened until it recovers.

<Callout type="info" title="Warmup and campaigns coexist">
  Keep warmup running even after you start sending campaigns. A mailbox that backs a live campaign keeps a small amount of warmup-style "health-check" traffic flowing so its reputation signals stay fresh while it does real work. Stopping warmup the moment a mailbox is "ready" throws away the reputation you built.
</Callout>

## Reply behavior

Real inboxes do not only send. They also reply. Warmup mimics this with a configurable reply rate per mailbox.

A portion of the time, instead of starting a new message, a mailbox will reply to an existing warmup thread it received. The reply stays on the same topic as the original message and threads correctly with a real `Re:` subject and reply headers, so the exchange looks like a genuine back-and-forth rather than a one-way blast. Replies are tracked separately in your warmup stats as a healthy-engagement signal.

## Pool safety and abuse protection

Shared warmup pools only work if every participant behaves. Warmbly continuously evaluates each mailbox's behavior and removes risky ones before they can harm the pool's reputation.

### Verification tokens

Every warmup email includes a hidden verification token. When the message is received, the token is checked. Missing, expired, forged, or malformed tokens are treated as suspicious and counted against the sending or receiving mailbox.

### Spam score and signals

Each mailbox accumulates a spam score from negative signals such as:

- recipients marking warmup mail as spam (user complaints)
- warmup mail landing in the Junk or Spam folder (placement)
- bounces, especially hard bounces
- invalid or forged warmup-token attempts
- tampering with delivered warmup mail

### Health states and auto-block

Based on those signals, every pool participant carries a health state. Warmbly uses these states to throttle or remove mailboxes automatically:

| State | What it means | What happens |
|-------|---------------|--------------|
| `healthy` | Normal behavior | Full warmup volume. |
| `watch` | Mild concern | Volume kept close to normal, spacing stretched modestly. |
| `throttled` | Probation | Volume cut and spacing widened until it recovers. |
| `quarantined` | Clear problem | Removed from the shared pool for a cooldown period. |
| `blocked` | Serious or repeated abuse | Removed for a longer period; requires review to return. |

A quarantined or blocked mailbox is not selected as a warmup sender or recipient, and it does not continue using the shared paid pool. Auto-block also triggers on clear abuse patterns, for example repeated invalid warmup-token attempts in a short window or a spam score that crosses the danger threshold.

<Callout type="warn" title="Acting early protects everyone">
  Warmbly intervenes well before a mailbox reaches the point where providers themselves would penalize it. A mailbox landing in spam a large share of the time has already become dangerous to the shared pool, so it is throttled or removed early rather than left to keep collecting negative signals.
</Callout>

### Getting back in

A blocked mailbox is not automatically restored just because time passed. Before it re-enters a pool, it should requalify: authentication still healthy, no recent complaints or hard-bounce spikes, no recent invalid-token attempts, and spam-folder placement back to a low level. When it does return, it re-enters gradually at low volume rather than jumping back to its old ceiling.

## Recommended posture

- Start new mailboxes at the default of around `10` warmup emails per day.
- Let the ramp climb slowly. Do not raise the ceiling casually above the default `40`.
- Keep warmup running alongside your campaigns, not just before them.
- Prefer more healthy mailboxes over forcing a few inboxes to send too much.
- Watch complaint and spam-placement signals more closely than open-rate vanity metrics. A low complaint rate matters more than maximum throughput.

## Related guides

<Cards>
  <Card title="Deliverability" href="/guides/deliverability">
    Authentication, suppression, and inbox-placement signals that warmup supports.
  </Card>
  <Card title="Personalization" href="/guides/expressions">
    Expressions for personalizing your campaign sends.
  </Card>
  <Card title="Mailboxes" href="/guides/mailboxes">
    Sending limits and provider setup for the mailboxes you warm.
  </Card>
</Cards>
