Explainer: Email Warming

What is email warming, and why it exists

Published: October 18, 2025

The war on spam gave us algorithmic guardians to protect our attention.
But these guardians created a new ritual for entry: a theater of fake civility, performed by machines for machines, before a single human is involved.

A fork in the road

For a business sending email, there are two realities. In the first, a new employee can send a handful of carefully crafted messages to relevant contacts and expect them to be delivered. In the second, the same act requires a weeks-long initiation: a ritual of simulated exchanges performed before a machine that must learn to trust you. The difference between these two realities is not how legitimate the business is, but how much email it plans to send. The practice that governs the second reality is known as email warming.

Sender reputation is ‘assumed guilty’

At its core, this divergence is a response to how modern email systems police their networks. Mailbox providers like Google and Microsoft have no history of a new domain’s behavior. Their algorithms rely heavily on sender reputation, which for a new domain is non-existent. Any early negative signals are catastrophic, as reputation is cumulative but hard to repair.

A sender’s score is built from a range of signals, including:

• Engagement metrics: opens, replies, and read times.
• Negative signals: spam complaints and high bounce rates.
• Technical authentication: SPF (verifies who can send from your domain), DKIM (signs messages so they can’t be altered), and DMARC (tells providers what to do if those checks fail).
• Volume and velocity: sudden spikes in sending activity are a major red flag.

Without a positive history, a high-volume campaign is immediately flagged as suspicious, condemning messages to the junk folder.

Email warming: a rehearsal for reputation

Email warming is the process of methodically building a positive reputation before launching a large-scale campaign.

It is, in essence, a rehearsal staged to train the inbox algorithms to trust you. Since the precise rules of the algorithms are opaque, the industry has reverse-engineered a set of best practices that imitate the signals of a trusted sender. This shared wisdom has crystallized into something of a ritual that mimics the behavior of a healthy domain over time:

• Start with a low volume of emails and increase it gradually.
• Send to a network of engaged inboxes that are programmed to open, reply, and mark emails as important.
• Maintain a near-zero bounce rate by using verified lists of these friendly inboxes.

Many modern sales tools — including Warmup Inbox, Lemwarm, Instantly, and Smartlead, to name just a few — automate this process, creating “reply loops” that simulate natural, two-way conversations and generate the positive engagement metrics algorithms look for.

Why new domains are required

Outbound teams rely on email warming because they rarely send from their primary company domain. The primary domain’s reputation is too valuable to risk on high-volume outbound. Instead, they register secondary domains (e.g., try-clean.com, get-outbound.io) specifically for outreach. Each of these new domains starts with zero reputation and must be warmed individually before being put into rotation.

The same is true for startups sending outbound for the first time. For them, the ritual is almost literal: a necessary initiation before they can reach a single real prospect.

A simple safeguard becomes the foundation for an entire manufacturing process.

The industry: outbound agencies and farms

As companies chased predictable pipeline, a parallel industry emerged to operationalize it. Outbound agencies and so-called “farms” manage outreach for dozens of clients at once, running hundreds of mailboxes across secondary domains. Each inbox must be warmed, maintained, and retired as reputation fades. A production line of synthetic correspondence. This isn’t a side effect of bad actors; it’s the logical outcome of trying to scale a behavior that was never meant to scale. In this economy, trust itself becomes an inventory that is generated, consumed, and replenished by machines.

In my experience since first stepping into a sales leadership role, roughly two out of every three unsolicited emails I receive come from these outbound farms, a statistic likely familiar to anyone with a public-facing role today.

The first Internet: a high-trust network

This entire cycle of algorithmic defense and circumvention stems from the original architecture of email. The underlying protocol, SMTP, was created by the same research community that built and ran the ARPANET — the embryo of the internet, a small high-trust network where participants knew each other by name — and was designed with no built-in identity layer.

(See The Day of the Blast for how that same network produced the first mass email.)

In response to the flood of spam that exploited this vulnerability, providers have layered on opaque, machine-learning filters to police the commons. If the rules for gaining trust were transparent, they would be immediately gamed. The result is a system where the only way to prove trustworthiness at scale is to successfully simulate it first.

The cost of scaling beyond consent

Ultimately, the need for email warming is a declaration of intent. A business that uses email as it was originally conceived, for relevant communication that people would expect to receive, has no need for it. Their natural, human-scale behavior builds reputation organically.

The warming ritual exists only to build one thing: a reservoir of algorithmic trust, to be spent on high-volume reach the moment the company decides to flip the switch. It is the rite of passage every company must pass through to scale beyond consent. The company pays the cost of this ritual only once. From then on, the burden is forever externalized, becoming an attention tax collected on everyone else.