Lately I've come across this problem at a few sites. OWA appears to be working fine, but access it somewhat erratic! Actually the problem varies a bit, with some users gaining access, and others either getting no access, or not able to see their inbox.

So why is it? Quite simply, it's highly likely your proxy server does not support WebDAV, the new HTTP protocol used by Microsoft Exchange OWA. Because the proxy does not understand the new HTTP protocol, it blocks the packets.

To see if your proxy is supported, take a look at the below URL that Microsoft refer to. (Interestingly enough ISA Server is not listed!)

http://www.webdav.org/other/proxy.html

There are a couple of ways you can get out of this problem, without the need to uninstall your proxy server, though it's not necessarily a solution.

The first method is to use a browser that doesn't support WebDAV. This could be Internet Explorer 4.x or Opera etc. If the browser doesn't support WebDAV, the server doesn't use this protocol, thus eliminating the problem of the proxy rejecting / blocking the packets. However, doing this, you loose out on a lot of the features of Exchange 2000 OWA.

The second method is a bit more involved. If you configure your server to use SSL with OWA (see my article on doing this - OWA and SSL), the problem may not be seen. This is because most proxy servers do not block these unrecognised requests as they do not decrypt HTTP packets encypted with SSL.