Why do mailbox providers care so much about engagement?

Mailbox providers care about engagement because it is the clearest signal of whether users find your mail valuable or disruptive. Opens, clicks, replies, and rescues from spam tell the system that your messages are welcome. Deletes without reading, ignoring messages for weeks, or marking them as spam signal the opposite.

Engagement is the closest thing mailbox providers have to a democratic vote. Their mission is to keep inboxes clean, safe, and useful. If users repeatedly ignore a sender, the filtering system learns that the content is unwanted and adjusts placement accordingly.

It is like watching the lanterns along the shoreline. When they light up in response to your ship, the harbor opens. When they stay dark, the harbor tightens its gates.