This refers to OWA 2003 access to freedocs, documents that are shared as standalone items in a folder.

The shared item is not an attachment to a post; rather, it is considered a "free document" or freedoc for short.Freedocs are not new to Exchange 2003; they are present in all prior versions of Exchange. However, Exchange 2000 made them more useful by making them accessible via OWA URLs (e.g., http://server/public/exchange2003/intro_e2k3.doc). Microsoft made a security-related change to Exchange 2003 that is blocking OWA access to freedocs.

There is a registry entry you can add to your OWA servers that re-enables OWA access to freedocs:

Location: HKLM\System\CurrentControlSet\Services\MSExchangeWeb\OWA
Value: EnableFreedocs
Type: REG_DWORD
Value: 0, 1, 2 or 3 (dec)

If this value is not present or set to 0, freedocs are completely blocked in OWA. This entry is not present by default, so freedocs are blocked out-of-the-box in OWA 2003. When set to 1, freedocs are only accessible when accessed directly via a back-end server; freedocs will not accessible to OWA users connecting through a front-end server. If you set the value to 2, freedocs are accessible from back-end servers, and from any front-end server configured with a Host Header entry that matches the following registry on the back-end server:

Location: HKLM\System\CurrentControlSet\Services\MSExchangeWeb\OWA
Value: AcceptedAttachmentFrontEnds
Type: REG_SZ
Value: comma-delimited list of FE servers
(e.g., fesvr1.domain.com, fesvr2.domain.com, etc)

Finally, when EnableFreedocs is set to 3, they are accessible to all OWA users.