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Apple develops software and services to sell more hardware. Mail is no different from other services in this respect. It’s a service that adds incremental value to the ecosystem, increasing the demand of Apple products.
Mail is a sweet deal for Apple. Mail provides significant utility to users, but Apple doesn’t have to pay the usual cost of running an email service (hosting, etc).
Mail Privacy Protection also plays an important role in Apples Privacy messaging strategy. This represents intangible branding value.