RFC 2919 - List-Id Header
Software that handles electronic mailing list messages (servers and user agents) needs a way to reliably identify messages that belong to a particular mailing list. With the advent of list management headers, it has become even more important to provide a unique identifier for a mailing list regardless of the particular host that serves as the list processor at any given time.
The List-Id header provides a standard location for such an identifier. In addition, a namespace for list identifiers based on fully qualified domain names is described. This namespace is intended to guarantee uniqueness for list owners who require it, while allowing for a less rigorous namespace for experimental and personal use.
By including the List-Id field, list servers can make it easier for mail clients to provide automated tools for users to perform list functions. The list identifier can serve as a key to make many automated processing tasks easier, and hence more widely available.
Helpful Information: http://www.ietf.org/rfc/rfc2919.txt