Just to view a different angle on this ive experienced the same problem where inside the LAN OWA is fine but externally over the internet the images all load, everything appears except the actual messages and you recieve the loading message and nothing else.
From my experience the problem has lied with a mixture of things, the most common seemed to be either browser related or transparent cache used by ISPs.
AOL users especially, or behind another corporate network behind certain squid / inktomi setups my users were having the exact problems you recieved.
In certain instances an easy way to get around this was to use an alternate browser. For example one member of staff was at a university, im unsure on their setup but the user could see their mail perfect simply by using netscape.
The other solution which was definately better in the long run was to implement SSL. There are obvious security benefits in this(OWA passwords not being sent in cleartext) and this solved all my issues.
As I mentioned, AOL for example use caching / proxies which as far as I know the end user could not turn off(never bothered to research as I cant stand AOL) Anyway since switching to SSL it worked perfectly from any location, using any ISP or any browser.
You dont have to pay for an SSL certificate, you can generate one for free using certificate services under win2k(although the users get a slight warning as its not from a 'trusted' source)
Personally I think you should be using SSL if using OWA over the public internet as opposed to tunneling in via VPN or similar.
Previous posts suggest authentication problems, I personally dont think this is the case as the users can actually authenticate and login as such, and see the OWA images / menus etc they just fail to see the inbox
Id suggest firstly trying a different browser externally and trying SSL(and allow port 443 to the OWA server rather than port 80)
I spent some time researching this, ill post the kb article that describes the problem, if I remember properly it references the technology used displaying the webpages, something to do with webdav.
Hopefully this solves your problem as it sounds identical to one ive had previously
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