---
title: "List Segmentation Claude Skill: Split a List by ESP"
description: "A Claude skill that splits a cold email list by who hosts each recipient — Google, Microsoft or a security gateway — then sizes the mailbox order for each."
url: https://warminboxes.com/skills/list-segmentation
breadcrumb: "Home > Claude Skills > List Segmentation"
---
Claude skill

# List Segmentation

Who hosts the people on this list?

Cold email lands better when the sending platform matches the recipient’s. Point this at a CSV and it works out who actually hosts every address on it, so the campaign can be sent from the right place instead of one pile of mailboxes aimed at everyone.

## What it runs

- **MX lookup per domain** — Who really hosts the mailbox, which is not always who the brand suggests

- **Gateway detection** — Proofpoint, Mimecast, Barracuda and friends, which filter before the mailbox sees anything

- **Deduplication** — A 10,000-row list is usually a few hundred unique domains

- **Order sizing** — Mailboxes and domains needed per segment, from the infrastructure planner

## What you get back

- The split as counts and percentages, with the MX evidence behind each call

- Which platform to send each segment from, and why

- Gateway-protected rows flagged separately, because mixing them in makes every other segment look worse than it is

- A mailbox count per segment

## Install it

One file, from GitHub. It calls the free WarmInboxes API, so there is no key and nothing to sign up for.

 mkdir -p ~/.claude/skills/list-segmentation
curl -sL https://raw.githubusercontent.com/Huz-ai-fa/warminboxes-claude-skills/main/list-segmentation/SKILL.md -o ~/.claude/skills/list-segmentation/SKILL.md

[View it on GitHub →](https://github.com/Huz-ai-fa/warminboxes-claude-skills/tree/main/list-segmentation)

## Then just ask

You do not name the skill. Its description tells Claude when it applies.

- "Split this lead list by email provider"

- "Are my prospects on Google or Microsoft?"

- "How many of these companies sit behind a spam gateway?"

Every answer cites the check behind it, so anything Claude tells you can be re-run by hand. A clean result is reported clean, with nothing to buy.

[All Claude skills →](https://warminboxes.com/skills)

## Do it by hand

The same checks as free tools in a browser.

- [ESP Segmenter](https://warminboxes.com/esp-segmenter)

- [ESP Calculator](https://warminboxes.com/esp-calculator)

- [CSV List Cleaner](https://warminboxes.com/csv-cleaner)

## The other skills

One page each.

- [Inbox Checker](https://warminboxes.com/skills/inbox-checker) — Is this domain burned?

- [Domain Audit](https://warminboxes.com/skills/domain-audit) — Is this domain set up correctly to send from?

- [Infrastructure Planner](https://warminboxes.com/skills/infrastructure-planner) — How many mailboxes do I need, and what does it cost?