Redirect is, as is says, a redirect. It's not a proxy. It means, you need to be able to access port 81 on your client, which may well be blocked in a large corporate network..
Sometimes Internet access is restricted on such corporate networks and you want to access your webmail over a regular port 80, which is normally not closed down. Hence the reason to use rewrite/proxy and no redirect. This post still does not answer the OP