RFC 2919: List-Id: A Structured Field and Namespace for the Identification of Mailing Lists

What this RFC defines

RFC 2919 defines the List-Id header, which provides a unique, structured identifier for a mailing list. The identifier takes the form of a domain-like string (list-name.list-host.example.com) that email clients can use to filter, organise, and label mailing list messages consistently across messages and providers.

Where you see it in practice

When your email client automatically applies a label or filter to all messages from a specific mailing list, it is reading the List-Id header defined by RFC 2919. Gmail exposes it directly through the list: search operator. The structured format allows clients to extract both a human-readable name and a unique identifier from a single header.

Need the normative text? The full specification is on rfc-editor.org. This page focuses on what the RFC does not show: real List-Id headers, how to read them in your inbox, and how to filter on them in each mail client.

How to find the List-Id of a message

• Gmail: open the message, click the three-dot menu, then Show original, and search the headers for List-Id.
• Outlook (desktop): open the message, then File, Properties, and read the Internet headers box.
• Thunderbird: open the message and press Ctrl+U (Cmd+U on macOS) to view the source.
• Apple Mail: View, Message, All Headers.

How to filter by List-Id in each mail client

Filtering on List-Id is more reliable than filtering on the From address, which can be the moderator, the digest system, or the individual poster depending on the message.

• Gmail: use the list: search operator, for example list:newsletter.example.com, then Create filter from the search options to label, archive, or skip the inbox.
• Thunderbird: Tools, Message Filters, New; in the condition list choose Customize, add List-Id as a custom header, then set List-Id contains newsletter.example.com.
• Outlook (classic desktop): create a rule with the condition with specific words in the message header and enter the List-Id value.
• Server side (Sieve):

require "fileinto";
if header :contains "List-Id" "newsletter.example.com" {
    fileinto "Newsletters";
}

Interaction with DMARC

Mailing list managers often need to rewrite the From header to avoid DMARC failures when the original sender domain publishes a strict DMARC policy. When the From is rewritten (typically to the list address with the original sender in the display name), the List-Id header remains unchanged, providing continuity for client-side filtering. This is one reason List-Id is preferred over From-based filtering in modern setups.

About the Author

Alaa · LinkedIn

Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.

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