The company apologized for Thursday’s breakdown without specifying how many people were affected. Twitter’s online messaging service was filled with complaints about the inconvenience.

Most users had no problem logging into their Yahoo email accounts.

ComScore Inc. says Yahoo Inc. had 284 million email users worldwide in March. That means hundreds of thousands could have been locked out of their inboxes, even if the trouble only affected a tiny fraction of Yahoo’s users.

Like most Web-based email providers, Yahoo offers its service for free as a way to give people returning to its website so the company can make more money selling ads.