When I implemented this, I ran into a nasty bug. Exchange, for some strange reason, looks up each email using the subject line (as opposed to say... THE FRIGGIN UNIQUE IDENTIFIER!). That means the email subject is part of the URL. If a percent symbol (%) is in the subject line of the requested email, Apache will complain that it cannot find the URL. This has to do with the way mod_proxy does encoding/decoding of URI escape sequences