This is how you can create default mailboxes automatically when using Dovecot and Plesk…

nano /etc/dovecot/conf.d/10-mailboxes.conf

And add the following…

namespace inbox {
  mailbox Drafts {
  special_use = \Drafts
  auto=subscribe
  }
  mailbox Trash {
    special_use = \Trash
    auto=subscribe
  }
  mailbox Sent {
    special_use = \Sent
    auto=subscribe
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
}

Then restart Dovecot…

service dovecot restart

Now when you add an email user in Plesk these mailboxes will be created automatically for them.