[imp] Problems getting imp directory to be the root folder accessed via web

[Eli]

I'm trying to make the entire horde directory invisible to anyone
accessing IMP, and I'm not sure if there's a proper way to do it or some
settings I need to change or something.

I put "Alias /webmail/ /opt/horde/imp/" in my Apache config (with some
Directory directives of course) so that I could just go to
http://server.com/webmail/, but when I do, I get an error saying that
"The requested URL /horde/imp/redirect.php was not found on this
server.".

I checked and the file does exist, so I assume it's some code in there
(part of the "smart" URL detection).  I just tried changing the
$conf['use_ssl'] variable from 2 to 1, and it made no difference, I
still get an error.

Any help would be appreciated.

Eli.