[squid-users] Problems with Squid and Squirrelmail

I apologise in advance for the lack of technical detail in this post,
but unfortunately I don't have a great deal of info regarding the
configuration of the problematic Squirrelmail site, and am not at
liberty to divulge any login details for testing.

The problem is as follows:
http://webmail.washington.k12.ok.us/ hosts a Squirrelmail installation
which is, for some reason, inaccessible through Squid (over plain HTTP)
- version 2.5STABLE12 and 2.5STABLE14 at least. Essentially, when
logging into the site, you are presented with a message claiming that
you have to log in to access the page. As far as I can tell from looking
at the traffic, the referrer, URL, and all cookies make it through the
proxy unmodified; as such, I am at a loss as to why the site is failing.
It does not fail when accessed directly, or through other proxy software
(ISA server, I believe).

I have encountered at least one form in the past that would similarly
fail for no apparent reason when accessed through the proxy, but this
was a custom web application.

What sort of things should I be looking for when troubleshooting this
sort of problem? Is this a common/known issue? (Can't find any reference
to other Squirrelmail problems, and I know of other installations that
work just fine.)

-- 
Philip Allison
Developer
SmoothWall Ltd. - http://www.smoothwall.net/
This email and any attachments transmitted with it are confidential to
the intended recipient(s) and may not be communicated to any other
person or published by any means without the express permission of
SmoothWall Ltd. Any views expressed in this message are solely those of
the author.  See http://www.smoothwall.net/emailnotice.html for the full
text of this notice.