The list-unsubscribe header is used by some email clients, including Gmail, to provide an unsubscribe header link within the email client. These unsubscribe header links are controlled by Gmail, and whether an unsubscribe header link displays is out of Click's or our customers' control. Gmail only renders the link for the domains that they deem have good reputation, which they evaluate independently.
All emails that you send with Click include a list-unsubscribe header ensuring that Unsubscribes are captured even when a recipient unsubscribes via Gmail's (or any other platform's) unsubscribe header link.
An unsubscribe header link will look something like this:
Recipients can click here to unsubscribe from your emails.
Here's a detailed explanation of what occurs when a recipient unsubscribes via the unsubscribe header link in Gmail:
1. The recipient clicks Gmail's unsubscribe header link.
2. Gmail will use the list-unsubscribe header to POST the recipient's information to Click's list-unsubscribe endpoint.
3. Click's list-unsubscribe endpoint receives Gmail's POST and treats it as a Global Unsubscribe: an Unsubscribe record is created for the recipient's email address and the Lead/Contact record's Bulk Email field is set to "Do Not Allow".