I do not want agent infrastructure tied too tightly to one inbox vendor.
The useful split is simple: identity belongs to the domain, mail hosting is replaceable. The public address should survive provider changes. Forwarding, aliases, and mailbox backends are implementation details.
This matters more for agents than for normal email because the address becomes part of the system boundary. It receives alerts, forwards tasks, signs up for services, and becomes a durable contact point for automation.
If the identity is portable, the rest can stay boring.