Problem to automate IE to POST form data

Hello,

Following the article Q167658 of MSDN, my IE Plug-in tries to navigate to
some web sites using saved data. The data was saved on BeforeNavigate2
event. Some web sites accept the posted request by the plug-in. But some
don't accept. I compared the data generated from the natural submitted form
from the web page and the data submitted by the plug-in. They are exactly
same.
So what could be the reason to cause the web sites rejecting the requests
submitted by the plug-in?

Thanks for any input.


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