Be notified of new mail in any Maildir inbox under ~/Mail, in the Zsh shell

I have a cron job that fetches new mail from several mail servers every 20 minutes. The mail is automatically sorted into Maildir mailboxes in a hierarchy under ~/Mail. I want to be notified in the shell if a new message has arrived in any local mailbox in the last 20 minutes. I'm using the zsh shell.