Yahoo Mail, which boasts more than 250 million users worldwide, hasn't been working for many of them today.
The company was vague about how many customers were hit by the outage, what the cause is, and how long the problem will last.
"Yahoo mail is currently inaccessible to some users," the company said in a statement. "We are working to correct the issue and restore all functionality immediately. We know that this may have caused some inconvenience and we apologize to our users who might be affected."
Spokesman Jason Khoury said the outage affected a "small subset of customers."
For the customers without mail, though, the outage trigged outrage, particularly on Twitter. Comments there ranged from the polite, such as @stickmantoo's "May I just say I'm not a fan of #yahoomail right now. #fail", to the more helpless pleading of @susanborst, who tweeted, "Come on #yahoomail...upgrading servers..."should clear up within a couple of hours"? #really?"
Add a comment if you are experiencing problems with Yahoo Mail.