Haven't consider this kind of functionality yet, but I think this is the kind of functionality we need in the product that is being built. I believe maildir is a better format for email cold storage. But ability to import .mbox - a real world need. Added it to the TODO list.

Current short term roadmap, with random priorities: - Outlook oath fix - Linux release - .mbox import - search improvements, to have more powerful search

Love your use case of replying to emails from the past.