Both Google and Yahoo mandate RFC 8058 one-click unsubscribe headers for all commercial bulk email transmissions. Senders lacking direct header-level unsubscribe triggers suffer automatic delivery penalties.
1. RFC 8058 Header Implementation Specs
✉️ Required SMTP Header Format
List-Unsubscribe: <https://mailcampaign.us/api/unsubscribe?token=abc123xyz>, <mailto:unsub@mailcampaign.us?subject=unsubscribe-abc123xyz>
List-Unsubscribe-Post: List-Unsubscribe=One-Click