Under the hoods, is your AI butter one service or many? It would be not-great for your weather or family-event-calendar-management components to communicate with each other or the orchestrator via email.
Receiving an email from the AI-butler rescheduling or relocating a planned outdoors family event because rain is expected would be excellent, using IMAP to wire-up the subcomponents together would not.
Who suggested using email in the service layer? I mean, you're not wrong, but this feels like you handed me a banana and then said I should have picked a better hammer.
We're talking about a conversation that has a human on at least one end, so email makes sense. For conversations involving no humans, of course there are much better stores and protocols if something like an asynchronous world-writable queue is what we want.
"Number of humans in the conversation" wasn't the distinction you initially established, I believe, but I wonder if it's closer to the one you had in mind.