(503) Server Unavailable Error

We have an application that loads XHTML documents that are authored by
end-users into a database. This database is used by my client's
Intranet to deliver a combination of static and dynamic content to
employees through my client's Intranet.

I did not write the original application that loads this content in,
but I have been tasked with making modifications to it and supporting
it. This application ran fine for several months, but one day we
started getting, "The remote server returned an error: (503) Server
Unavailable", every time we loaded a document.

The code that was blowing up was pretty simple:

XmlDocument xmlDocument = new XmlDocument();
xmlDocument.Load(filePath);

filePath was the full path of the XHTML file, and we were blowing up
on the Load() method.

After some investigation, I determined that the DocType line in the
XHTML we were loading was causing the issue:
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

I found that if I put the w3.org address into Internet Explorer, I got
the same (503) Server Unavailable error as our application was
getting. Strangely enough, I did not get this error from FireFox.
To resolve the error, I used FireFox to pull the
xhtml1-transitional.dtd file down locally from w3.org, and the 3
associated files:
xhtml-special.ent
xhtml-symbol.ent
xhtml-lat1.ent

I put these files onto our web server, (www.myclient.com/Intranet/XML)
and changed the DOCTYPE line in the XHTML content to this:

"http://www.myclient.com/Intranet/XML/xhtml1-transitional.dtd">

This resolved the issue.

So why were we getting this "The remote server returned an error:
(503) Server Unavailable" error?

It turns out that w3.org was blocking requests from my client's external web server. Here is
an explanation:

http://www.w3.org/blog/systeam/2008/02/08/w3c_s_excessive_dtd_traffic