My guess is that Twitter uses the Google SafeBrowsing API, in addition
to other blacklist APIs.

http://code.google.com/apis/safebrowsing/

Google SafeBrowsing is basically two databases, which you can host
locally, and are constantly updated by Google. One database consists
of potential phishing addresses and the other one contains potential
malware addresses.

You do lookups against these databases. You don't visit the actual
destination site. The same applies to other blacklist databases.

That's why there is no need to check for a tco user agent or IP
address. Tco never visits or crawls your site. It simply redirects the
user's browser to your site.

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