Can you elaborate a bit how your mailbox provider knows about click/engangement rate? E.g. why would Fastmail have any metrics on how their users interact with

Can you elaborate a bit how your mailbox provider knows about click/engangement rate? E.g. why would Fastmail have any metrics on how their users interact with the mail they receive? I see everybody calling out that one should keep a clean subscriber list (e.g. only keep engaged users) but I fail to see this is relevant to the actual mail acceptance/inbox delivery.