A List-ID header is an additional header included in a message that transmits a unique mailing list identifier. It is not displayed by default in the email client, which uses the information contained in the header to filter and sort incoming messages.
One email client, in particular, Gmail, uses the list-ID to sort messages by type and display them in different tabs. Messages with custom headers, including unsubscribe headers, are sorted out of the Primary inbox and displayed under the Promotions tab. However, if the user chooses to move a message from the promotional area to the primary inbox, all future messages sent containing the same list ID will also be displayed in the Primary tab.
A header has a name and a value with a specific syntax for each component, in this case:
Description <List-ID>. These components are to be formatted as follows:
Description: The description before the identifier is optional and is written as
List-ID:[phrase]. The description is used as a plain text name for the list.
List-ID: The List-ID is built using the domain name plus a string that uniquely identifies the list, written as
Customized list IDs
The following guidelines apply to all customized list IDs:
The domain name used inside the angle brackets is subject to the same restrictions that apply to domains used as the From Address for sendout. The system will not prevent you from generating a list ID using a domain name for which you do not have authorization. However, it is important that you use only domains verified for your use in Mapp Engage. For more information, see Domain Verification.
The angle brackets surrounding the List-ID are required.
In an example using Newsletter for both the description and the label, the list-ID header would appear as follows: