Yahoo Mail has experienced an extended outage this week and is now in its fourth day. But Yahoo refuses to say just how many users are being affected.

On Wednesday, a Yahoo blog post said that "some of our users haven't been able to access their mail since 10:27 PM PT on Monday night" because of a hardware problem that was "harder to fix" than they expected. An update later Wednesday described the group as " a small percentage of our users."

But Yahoo's e-mail service has around 100 million active users, so even a "small percentage" could mean that millions were left without access to their e-mail. And Yahoo has been less than forthcoming about how many total users have been affected by the outage. In a phone interview Friday morning, a Yahoo spokesperson repeated the "small percentage" line and declined to give a specific figure.

It does appear that the company has been making some progress toward returning service: The Yahoo spokesperson told The Post that "almost all affected users now have access to their e-mail" and that the company was "very sorry about" the outage. In an update Thursday at 11 p.m. Pacific Time, Yahoo said that "most of the affected Yahoo Mail users should have access to their accounts on the web, POP and through our mobile apps," but access through another protocol, IMAP, was "not yet widely available."

Will Oremus at Slate's Future Tense blog argues that this could be a Waterloo moment for CEO Marissa Mayer's attempts to revive Yahoo: A multi-day outage is a major disruption, especially to a service like e-mail, which many users rely on for things tied to their physical lives, like paying bills. Combined with the user backlash over Yahoo Mail's redesign this year, they could be alienating some of their most loyal customers.