How Email Warmup Works: From Zero History to Stable Inbox Placement

When you launch an email campaign, your goal is straightforward: maximize open rates and convert as many prospects as possible. For that, you spend considerable time crafting relevant copy and optimizing the email for structure, formatting, and timing.

However, the moment you hit “Send,” control shifts. It’s the mailbox providers who decide whether your message gets delivered, filtered, or blocked. And if you’ve created a new domain or inbox, they’re likely to treat it with caution as they have no prior data about you.

A sudden surge in volume, long gaps between sends, and low recipient engagement all signal to mailbox providers that your traffic poses a risk. That judgment forms fast, and it influences every campaign that follows.

Industry benchmarks show global inbox placement rates dipping below 80%, meaning roughly one in five marketing emails fail to reach the primary inbox when sender signals slip.

Email warmup exists to prevent this outcome. In this blog post, we’ll discuss what it is, how it works at a system level, and how you can use it to protect inbox placement.

What is Email Warmup?

Simply put, email warmup is the process of gradually increasing sending volume while generating consistent, positive engagement from mailbox providers such as Google (Gmail, Google Workspace) and Microsoft (Outlook, Office 365).

It typically begins with a small number of emails and ramps up steadily across days or weeks, allowing filtering systems to evaluate engagement and trust signals.

Manual vs Automated Email Warmup

You can warm an inbox manually by writing and replying to emails yourself, or you can use an automated system that runs the process continuously. Both approaches are fine, but they behave very differently in practice:

Why Email Warmup is Critical for Deliverability

A brand-new domain or inbox has no historical sending behavior. When you send your first campaign, mailbox providers assume you have nothing to compare it with. Email warm-up matters for deliverability because it:

1. Creates a stable foundation for growing outbound safely

Without warmup, your earliest outreach becomes the model that providers learn from. Low opens, delayed replies, and uneven volume are treated as your natural behavior.

Warmup ensures your inbox already has history. Providers evaluate your first campaigns against an existing pattern instead of treating them as a cold start. That reduces the chance that normal outbound activity is classified as spam, automated bulk sending, or account misuse.

2. Concentrates positive signals when they carry the most weight

In the absence of history, every open and reply is amplified. Early engagement disproportionately shapes how aggressively future mail is filtered. Warmup front-loads positive interaction into this sensitive period. Providers see threads initiated, messages opened, and replies returned before any campaign pressure exists.

3. It stabilizes deliverability as you scale

Reputation is not created once. It is recalculated continuously. By the time real outreach begins, your inbox is already classified as predictable and responsive. As volume increases, providers can measure change against a known pattern. That stability is what allows you to grow without triggering sudden drops in placement.

Signals mailbox providers learn from

How the Email Warmup Process Works

Here’s a quick breakdown of how inboxes warm up:

1. Initiates low-risk, human-patterned sends

Email warming starts with a minimal number of emails sent from your inbox. These messages follow normal sending patterns and are delivered like everyday email.

From the provider’s point of view, at this stage, you’re a low-activity sender beginning a few conversations.

The messages are also spaced across the day rather than sent in bursts. The timing follows a pattern similar to how people usually send mail. Nothing about the transport behavior looks automated or abrupt.

2. Creates engagement loops

A short delay in email engagement is introduced to mirror human behavior. After that pause, it’s processed as a real recipient would: it may be opened, replied to, archived, starred, or moved to another folder.

At this stage, each thread completes a loop. You send a message. It gets delivered. The recipient interacts with it inside the provider’s environment. That loop produces the first usable data about how your mailbox behaves.

3. Increases volume through repetition

Once the send and engagement pattern remains stable, you can slightly increase volume the next day. The change is narrow and continuous. There are no jumps and no compressed send windows. Every additional message follows the same delivery and interaction path. Each day adds data of the same type.

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4. Builds a behavioral history with mailbox providers

As this cycle repeats, providers stop treating your emails as isolated actions and start recognizing a consistent sending pattern. Your email address develops a behavioral history.

It appears as a sender that initiates conversations, receives replies, and remains in inbox folders. At this level, sending patterns stabilize. Messages from this address look predictable and no longer trigger abuse thresholds.

5. Monitors performance signals and adjusts pacing

Even after you introduce real outreach traffic, the same mechanics apply. Your inbox now produces both intentional sends and warmup traffic. Both originate from the same sender and are evaluated by the same systems.

The warmup traffic maintains a baseline of positive engagement. It smooths the statistical profile of your sender, while cold emails introduce variance.

If your email campaign performance degrades, the system detects changes such as delayed delivery or reduced inbox placement and adjusts its internal pacing. This prevents sudden reputation collapse.

Bonus read: Warm Up Your Email Domain the Right Way

Warmup Strategy for Cold Outreach Teams

Learn how to introduce new domains and inboxes into your outreach system without triggering provider risk models.

Each step below maps to a specific behavior mailbox providers monitor: how new senders appear, how volume changes, and how engagement evolves.

1. Sequence domain setup before campaign creation

Start with the domain. After it’s registered, create your inboxes and connect each one to your warm-up system before any campaigns begin.

For the next fourteen days, each inbox should send a small, steady volume of warm-up emails every day and receive real opens and replies. Don’t build sequences yet or attach these addresses to any campaign.

Before launching a campaign, run a spam test to confirm inbox placement. You can use MailReach’s Email Spam Tester to send a live message from your inbox to a distributed set of real mailboxes across major providers.

This test is typically run toward the end of the warmup period, such as Day 13 or Day 14, to verify that nothing will hurt inbox placement once outreach begins.

Each test shows you:

• Inbox vs spam placement per provider
• Provider-specific filtering behavior
• Authentication and configuration issues
• Content and structural triggers
• A consolidated spam score based on real delivery outcomes

When an email lands in the primary inbox across Workspace and Microsoft 365 for several consecutive days, typically three to five, it can be deemed ready.

2. Warm every sending identity

Now, it’s tempting to assume that an established domain makes new addresses safe. In practice, new addresses do benefit from domain-level trust, but they still need their own engagement history.

Any inbox used for campaigns must generate enough positive interaction to support its outbound volume. Warm-up provides that baseline engagement, so campaign sends don’t overwhelm the signals mailbox providers observe from each address.

For that reason, every new sending inbox should be warmed before it’s attached to outreach, following the same progression outlined earlier.

Create → Warm → Placement stabilizes → Attach to outreach

3. Run warmup in parallel with outbound

Prospect behavior is uneven by nature. Some days, replies arrive in clusters. Other days, nothing lands at all. So if warm-ups stop when campaigns go live, those swings become the only pattern your inbox shows.

That’s why you should keep a consistent layer of warm-up traffic running on every sending inbox. For example, if an inbox starts with 20 cold emails per day, it continues to send 15–20 warm-up emails alongside them.

And when you increase campaign volume to 40, that warm-up layer doesn’t scale with it. The inbox sends 40 campaign emails plus the same warm-up traffic as before, continuing to generate opens, replies, and normal mailbox interactions in parallel.

This keeps both traffic shape and engagement signals stable as volume grows. Mailbox providers continue to observe predictable, human-like behavior instead of abrupt shifts driven solely by outreach.

Build Sender Reputation Through Continuous Email Warm Up

Sender reputation isn’t created once. In fact, it’s made, maintained, and repaired over time. For a new domain, that process starts before your first campaign. Most teams need around two weeks of warm-up before an inbox is ready to send.

Once campaigns begin, the warm-up must continue. If it stops, engagement drops. When engagement drops, reputation follows. Cold outreach alone doesn’t generate enough positive interaction to sustain inbox placement. 

This is why warm-up belongs in all three places: before, during, and after campaigns. It’s a natural part of how email works now.

MailReach’s email warm-up tool automates that cycle. When you connect an inbox, it sends emails from that address into a network of more than 30,000 high-reputation Google Workspace and Microsoft 365 mailboxes.

Those messages land in real provider environments and are opened, replied to, starred, and removed from spam, producing the same engagement signals mailbox providers use to score your campaigns.

MailReach controls pacing at the sender level, ramping volume in small increments up to 45 warm-up emails per day and increasing it when a reputation needs repair.

The platform notifies you when your Reputation Scores drop or improve, through Slack or webhooks, so you can respond before placement degrades. Start warming up your emails with MailReach today!