List-ID Headers

General Information

The List-ID header is a specialized header included in email messages to provide a unique identifier for a mailing list. This header is essential for email clients, enabling them to filter and organize incoming messages effectively. Although the List-ID header is typically hidden from users, it plays a critical role in email management.

Purpose of the List-ID Header

Email clients, such as Gmail, use the List-ID header to categorize emails into different sections, such as Promotions, Social, and Primary. For instance, emails with custom headers, including unsubscribe headers, are often directed to the Promotions tab. If a user manually moves an email from the Promotions tab to the Primary tab, future emails with the same List-ID will automatically appear in the Primary tab.

List-ID Syntax

The List-ID header is composed of two main components, a description, and a value, structured as follows:

1. Description (Optional): A text phrase that provides a plain text name for the mailing list. This appears before the identifier.

2. List-ID (Mandatory): Formatted as <[list-label].[domain]>, where:

   • list-label: A unique string that identifies the mailing list.

   • domain: The domain name associated with the mailing list.

Example:

For a newsletter, both the description and the label might be "Newsletter," resulting in the following List-ID header format:

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

This format ensures that email clients recognize and correctly filter messages associated with this mailing list.

Creating Customized List-IDs

When setting up customized List-IDs, it is important to follow these guidelines:

1. Domain Name Restrictions: The domain used in the List-ID must comply with the same restrictions as those applied to domains in the "From" address of the email. Ensure only verified domains are used, especially when sending through systems like Mapp Engage.

2. Angle Brackets: The List-ID must be enclosed in angle brackets < > to ensure proper formatting and recognition by email clients.

By adhering to these guidelines, you can optimize the effectiveness of your email communications and ensure they are properly categorized by email clients.