About

Cold email you can read, run and trust.

Warmbly is the open-source platform for email warmup and cold outreach. Everything that decides whether your email lands in the inbox is in a public repository, and you can run all of it yourself.

Read the codeRead the story

Why Warmbly exists

For years the main tools for cold email have charged by the mailbox while keeping their warmup logic, sending rules and abuse thresholds closed. You paid for volume. You never got to see how decisions about your reputation were made.

We started Warmbly to take that away. The workers that send your mail, the warmup pools, the encryption and the health rules all live in a public repository under Apache 2.0.

So when Warmbly slows a mailbox down, the dashboard says why in a full sentence, and the sentence is written by code you can open. The figure beside this is a real one: the reason a throttled mailbox shows, and the lines of service.go that wrote it.

Founders, agencies, sales teams, recruiters and fundraisers can read the engine, run it on their own servers if they want, and stop paying a tax on every mailbox they add.

We are a small team building in public. We say what we are working on, we don’t invent numbers, and the product gets better where everyone can see it.

When deliverability breaks on a closed platform, you can’t read the code that broke it. You can only write to support and wait.
A mailbox slowed for spam placement, and the rule that slowed it.

What we believe. Six rules the code is held to.

These aren’t values for a slide. They are written into the codebase, and a feature that breaks one doesn’t ship. Each pair below sits next to the code or the record that proves it.

In the open
  1. 01

    Open by default

    The whole platform is on GitHub under Apache 2.0: sending, warmup pools, health thresholds and abuse checks. If you want to know why a mailbox was paused, you can read the code that paused it.

  2. 02

    Built in public

    The changelog is public and dated, and every entry maps to a merged change. We don’t invent metrics or pretend to be bigger than we are.

Safe by default
  1. 03

    Safety is per mailbox

    A safe day of sending is the sum of what each mailbox can safely send. We will not ship a feature that pushes a mailbox past its own limits to make a chart look better.

  2. 04

    Act before the providers do

    Our thresholds for the shared warmup pool are stricter than the points where Google or Amazon SES start penalising a sender. By the time a provider would act, the mailbox has already been slowed down.

Nothing hidden from you
  1. 05

    Encrypted by design

    Mailbox credentials are encrypted with a dedicated credentials key, and message content with a data key that belongs to your workspace alone.

  2. 06

    No black boxes

    Every health state, suppression and pool block comes with a reason you can read in the dashboard, and the rules behind them are documented.

How it’s built. One part decides, the other part sends.

Adding capacity means adding small workers, not growing one giant server. Your mailbox provider delivers the mail, so a worker is only where your mailbox signs in from, and Warmbly keeps that stable.

The control plane decides

The backend and consumer hold your data, decide which mailbox sends what and when, and assign each mailbox to a worker.

The bus carries the work

Commands go out on a topic per worker and results come back on one shared stream, over NATS or Kafka.

Workers send and sync

They sign in to your mailboxes, send, sync and report back. They hold no database connection, so one that goes away is simply replaced.

What’s different. An open engine, in a category of closed ones.

WarmblyClosed-source tools
Source codeApache 2.0 on GitHub. Read it, fork it, run it.Closed. You see the dashboard, not the engine.
Where it runsOur cloud, or your own servers with one command.Their cloud only.
Health rulesDocumented, and stricter than provider enforcement.Undocumented until something goes wrong.
ChangelogPublic and dated, one entry per merged change.Announcements on a marketing schedule.
PricingPer workspace, with unlimited mailboxes and warmup.Often per mailbox or per seat.

This describes how Warmbly is built. We don’t speak for any other company’s plans or pricing.

The repository. What you’ll find when you open it.

warmbly/warmblymain
  • cmd/backendThe API and the business logic. Owns the database.
  • cmd/consumerFolds results from the workers back into the platform.
  • cmd/workerSends and syncs mail. Holds no database connection.
  • cmd/cliThe warmbly command-line tool.
  • tracking/The open and click tracking service, in Rust.
  • realtime/The live websocket layer for the dashboard, in Elixir.
  • web/The dashboard.
  • site/This website.
  • docs/The documentation at docs.warmbly.com.
Documentation Changelog Security and trust Discord

Read it, run it, or let us run it.

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