List-Unsubscribe header
Compliance & List Hygiene · Critical priority · hard-fail gate
What it is
Bulk/marketing mail to Gmail/Yahoo must carry a List-Unsubscribe header with an HTTPS URI (ideally plus mailto).
How to fix it
Add a List-Unsubscribe header containing an https URL (and optionally a mailto:).
This is a hard-fail gate. If it fails, Gmail / Yahoo / Outlook may junk or reject your mail regardless of everything else — so fix it first.
Check your email
See how your real email scores on this and every other deliverability factor.