You should register the HTTP handler both in the <handlers>
sections of web.config in order to ensure that it will work both in IIS6 and IIS7. You can check this setup in the help article Getting Started
of RadCaptcha. Note that you have to use a fully qualified name if the Telerik.Web.UI assembly is installed in GAC, which is covered in the linked article as well.
the Telerik team
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