Yahoo confirmed reports that it is preventing some Yahoo Mail users from seeing their own e-mails until they turn off their ad-blocking software.

Several reports have indicated that some U.S. users have tried to view their mail, only to receive a message from the site asking them to turn off their ad-blocking software first. In a statement to The Washington Post, a spokeswoman 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."

The move could be seen as a reaction to the growing use of ad blockers, which people are installing on their devices to get away from annoying, sometimes invasive, often inconvenient advertisements. That's spelled some trouble for Internet companies such as Yahoo and Google, as well as media companies, all of which depend on advertising revenue to fund their businesses.  The debate around ad-blocking has kicked into high gear in recent months, particularly after Apple started offering support for ad-blocking in its mobile Safari browser for the iPhone and other iOS devices.

The move has served as a wake-up call for some in the advertising industry. The Interactive Advertising Bureau said in an October blog post that it would spearhead an effort to make ads that load more quickly, are more secure and are more respectful of users' privacy choices.

Yet while some firms -- notably, The Washington Post -- have put up messages asking users to turn off their ad-blocking software in exchange for access to content, Yahoo may stand alone in actively blocking users from reaching their own correspondence.

The move may bring one thing into sharp focus: When it comes to free services, companies have to balance the needs of their users with the needs of the advertisers who pay the bills. And while the Yahoo test is still only a test, the company certainly seems willing to explore options at all points on that spectrum.

Yahoo is well within its rights to do so, said Ansel Halliburton an attorney at Kronenberger Rosenfeld who specializes in Internet law. After all, it's not as if the company is violating any part of its legal agreement with consumers by making this request; in fact, in some cases, consumers may be in breach of contract for using ad-blockers.

"A lot of times when I write terms of service now, I put that in," Halliburton said. "Practically speaking, no one is going to sue 5 million people for using an ad blocker, but it gives them some leverage to block the blockers."

Yahoo in particular doesn't appear to have language specifically prohibiting users from using ad blockers in its terms of service, though agreeing to use its services does mean that users must agree to the statement "Yahoo may include advertisements and that these advertisements are necessary for Yahoo to provide the Yahoo Services."

While users can always choose to vote with their feet if they don't like what Yahoo or others may do in the future, that hardly means this debate will end any time soon. Halliburton said that he expects the escalation in the battle against ad-blockers will continue. He noted that some companies in Germany have already tried -- and failed -- to challenge the legality of blockers by filing suit against the parent company of AdBlock Plus. Companies could say that blocking ads is a violation of copyright, as broadcasters tried to do when television commercial-skipping technologies first popped up, he said.

That, of course, does not change how Yahoo's experiment may affect its public reputation. People have been vocal about their displeasure with the test, which comes at a time when Yahoo faces serious questions from analysts about its ability to draw and retain users.

"PR-wise, this seems like a harsh thing to do," Halliburton said. "But running e-mail infrastructure at that volume takes a lot of resources."