Google, Microsoft, and Yahoo have bots that belong to their contextual ad programs.

These bots spider web pages to understand what the page is about so they can match appropriate advertising.

If any publishers are blocking these bots they will keep themselves from having the proper (or in some cases, any) ads show on their website.

Block these bots (if you’re showing contextual ads on your site) and you will lose money.

The useragents not to block:

Mediapartners-Google
MSNPTC
YahooYSMcm

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