What is the economic incentive for mailbox providers to filter spam?

Mailbox providers run enormous infrastructures. Processing billions of messages daily cost real storage, bandwidth, CPU, and support resources. Spam consumes these resources without providing user value.

Filtering spam reduces operational costs, protects users, and improves platform trust which keeps customers loyal. On consumer services this also preserves advertising value because users stay active on the platform instead of abandoning cluttered inboxes.

It is like maintaining a clean harbor. If the waters fill with debris, ships stop visiting, trade slows, and the entire port economy suffers.