list_unsubscribe is currently available only from Professional. I'd like to ask you to consider exposing that single fieldon every plan, including Hobby — separately from the general custom-headers feature. The reason custom headers are gated is presumably abuse: header injection, forged Message-ID or Received lines. None of that applies here. list_unsubscribe is one standards-defined field (RFC 2369 / 8058) whose only possible effect is to make it easier for a recipient to stop receiving mail. It reduces complaints rather than enabling them — which is in your interest as much as mine, since a recipient who can't find an unsubscribe route presses the spam button instead, and that complaint lands on your shared pool. It has also stopped being a premium capability. Both Microsoft and Google name one-click unsubscribe (RFC 8058) in their published sender requirements. As it stands, a Hobby customer is advised to improve deliverability while being denied the one mechanism those requirements name explicitly.
