mtt.register("migrate v1", function(callback)
    local entry = mail.get_storage_entry("old_v1_player")
    assert(entry)
    assert(#entry.inbox == 1)
    assert(entry.inbox[1].from == "singleplayer")
    assert(entry.inbox[1].to == "old_v1_player")
    assert(entry.inbox[1].subject == "test1")
    assert(entry.inbox[1].body == "test2")
    assert(entry.inbox[1].id)
    assert(entry.inbox[1].time > 0)

    callback()
end)

mtt.register("migrate v2", function(callback)
    local entry = mail.get_storage_entry("old_v2_player")
    assert(entry)
    assert(#entry.inbox == 1)
    assert(entry.inbox[1].from == "someone-else")
    assert(entry.inbox[1].to == "old_v2_player")
    assert(entry.inbox[1].subject == "test1")
    assert(entry.inbox[1].body == "test2")
    assert(entry.inbox[1].id)
    assert(entry.inbox[1].time == 1678467148)

    callback()
end)