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For example, Company A has a Yahoo! feed. They give their feed out to thousands of sites. All the clicked ads on those sites redirect through Company A (thus making it impossible to see where the clicks came from in your stats). Blocking Company A in your blocked domain tools will not block any of the sites that were given the feed. I do not know if this issue is isolated to just a few major domain lander companies or all.
This essentially is a huge loophole in the tool that can be used by fraudsters. Yahoo! was eager to have the loophole shown to them. The issue was passed through to the technical team weeks ago and has yet to be resolved. I would recommend taking a look at your stats to see if you are still receiving traffic from domains that you have already blocked. If so, it may be in your interest to request a refund for those clicks.