Function conversations 

fn conversations(
    cards: &[Card],
    sent: &[Card],
    shown: &dyn Fn(&Card) -> bool,
    item: &dyn Fn(&Card, bool) -> MailItem,
) -> Vec<MailItem>

Messages by conversation: each one’s newest message of the folder first, the conversations by their last message (yours included), then the rest of each, oldest first, your answers from Sent among them. A conversation shows when its first row would.