They know I'm using a proxy(Tor)...but how?

["gregery" <gregeryhouse69@xxxxxxxxxxx>]

Hey all,

I am trying to register at a website forum and I get an error message
when I try to register.  The message basically says that it seems I am
using an anonymous internet connection or a proxy.  The site is not
blocking Tor by exit-node because I get the same error message when I
try to register at the site while using a public high-anonymous elite
proxy (L1).  I think the forum administrator is using headers to decide
if a new member is using a anonymous connection or not.  

I use the current TorBrowser Bundle with current TorButton and
RefControl to spoof the referrer headers.  I also use vanilla Tor,
Vidalia and Privoxy with the same result.  I tired spoofing my
user-agent away from the default TorButton U-A settings, I spoofed as a
Mac, Firefox, IE, Opera, etc, all for not.

Does anyone know how I can circumvent this block?  I at least would like
to know what in my headers is setting off red flags as that means all
other Tor users in my anonymity set are potentially setting off red
flags too.

Thank you
-- 
  gregery
  gregeryhouse69@xxxxxxxxxxx

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