It's not easy running an online e-mail service with hundreds of millions of users.
Later on the same day that Yahoo Mail wasn't available to a small fraction of its users, Hotmail had troubles with disappearing in-boxes. Here's the company statement:
"On Thursday evening, there was a short-term service disruption which prevented Windows Live Hotmail customers worldwide from accessing their in-box for approximately two hours. Microsoft worked quickly to restore access. No data was lost during the outage. Microsoft apologizes for any inconvenience this may have caused." The company didn't share details about how many people were affected.
E-mail is central to the lives of a growing number of people, but nobody's perfect when it comes to offering the service. Comcast had an e-mail outage on Saturday, and Google's Gmail went down in February.