---
title: "List-ID Headers"
slug: "list-id-headers"
updated: 2026-01-08T16:23:00Z
published: 2026-01-09T15:00:07Z
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mapp.com/llms.txt
> Use this file to discover all available pages before exploring further.

# List-ID Headers

## Overview

List-ID headers are specialized email headers that uniquely identify a mailing list. They are primarily used by email clients to recognize, filter, and organize messages that belong to the same list. Although end users typically do not see this header, it plays an important role in how inboxes categorize and handle incoming emails.

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## Key Characteristics

- **Purpose**: Helps email clients group and categorize messages from the same mailing list, for example, into tabs such as Promotions, Social, or Primary.
- **Client behavior**: If a recipient manually moves an email with a specific List-ID to another inbox category, future emails with the same List-ID are automatically placed there.
- **Visibility**: The List-ID header is usually hidden from recipients but is processed by email clients in the background.

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## List-ID Syntax

The List-ID header consists of two parts:

1. **Description (optional)** A plain text name that describes the mailing list. This part appears before the identifier.
2. **List-ID (mandatory)** A unique identifier in the format `<list-label.domain>`, where:
  - `list-label` is a unique string that identifies the mailing list.
  - `domain` is the domain associated with the mailing list.

**Example** For a newsletter, both the description and the label might be “Newsletter”:

`List-ID: Newsletter <newsletter.companydomain.com>`

This structure ensures that email clients can correctly recognize and process messages from the same mailing list.

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## Configuration and Behavior

When creating customized List-IDs, the following rules apply:

- **Domain restrictions**: The domain used in the List-ID must meet the same requirements as domains used in the **From** address. Only verified domains should be used, especially when sending through systems such as Mapp Engage.
- **Formatting**: The List-ID value must always be enclosed in angle brackets `< >` to ensure correct recognition by email clients.

Following these guidelines helps ensure that emails are reliably categorized and consistently handled by recipients’ inboxes.

## Related

- [Domain Verification](/domain-verification.md)