What is a List-Unsubscribe Header?
List-Unsubscribe is an optional email header that allows email recipients to opt-out of receiving emails without clicking through an unsubscribe link or marking a sender as spam or junk.
It is displayed next to the email sender information.
Why is it important?
List-Unsubscribe Header is a user-friendly way to offer an opt-out for your recipient. Thus, they won't have to search for a unsubscribe link in your email body.
Offering such feature in your emails is a good way to avoid that your recipients mark your email as spam to not hear from you again. Which is very damageable to your email deliverability.