Yahoo has sparked controversy by preventing some users from accessing their email accounts if they have an ad blocker installed. It's something that seems to be restricted to the US at the moment, and Yahoo says it is testing out the block on a "small number of Yahoo Mail users".

Edward Snowden recently said that people have a duty to use an ad blocker, and it seems that nearly 20 percent of web users have followed his advice. Just like Google, Yahoo makes a large proportion of its income through advertising, but affected Yahoo Mail users feel that the company is holding their email accounts to ransom.

The Adblock Plus support forums features posts from users who found themselves locked out of their account, and there were also complaints on Twitter. People are more than a little disgruntled by the move, but some users have noted that by switching Yahoo Mail to Basic view, it is possible to get around the block.

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In a statement, Yahoo spokeswoman Anne Yeh said:

At Yahoo, we are continually developing and testing new product experiences. This is a test we're running for a small number of Yahoo Mail users in the US.

When an ad blocker is detected, Yahoo blocks access and the URL has ADBLK_TRAP added to the end as a fairly clear indicator of what is going on. Has your account been hit?

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