What Is List-Unsubscribe?
TL;DR
List-Unsubscribe is an email header field that gives the receiving mail client a machine-readable way to opt a recipient out, published as an HTTPS URL, a mailto address, or both. Inbox providers render it as a native unsubscribe control next to the sender name, which gives an irritated recipient an alternative to the spam button.
What goes in a List-Unsubscribe header?
One or more URIs in angle brackets, comma separated. RFC 2369 introduced the field alongside the other List- headers, and it accepts a mailto address, an HTTPS URL, or both, with the receiving client free to choose which it uses. A mailto target has to be a mailbox you actually process, not a placeholder.
The header on its own is a hint rather than an automated opt-out. RFC 8058 adds the pair of headers that make a provider act without the recipient leaving the inbox, which is what Google and Yahoo ask bulk senders to implement.
Should cold outreach include List-Unsubscribe?
For anything that resembles a campaign, yes. The Gmail requirements attach one-click unsubscribe to marketing and subscribed messages from bulk senders rather than to genuine one-to-one mail, but the reason to include the header is independent of the requirement: it is the cheapest available way to turn a would-be complaint into a suppression.
A visible opt-out line in the body is worth keeping alongside it. The header is rendered by some clients and not others, so a recipient using a client that hides it needs something to click, and CAN-SPAM addresses the body rather than the headers.
Frequently asked questions
It increases recorded opt-outs and reduces spam complaints, which is a trade worth taking. The recipient who uses the header was leaving either way; the difference is whether that decision arrives as a suppression you own or as a report to the provider that filters you.