According to Mayer, the problem was traced back to a specific hardware outage in one of Yahoo's storage systems serving 1 percent of its users. At the time, Yahoo expected to have things fully operational by 1:30 PM PT on Tuesday, less than 24 hours after the outage first began.
"However, the problem was a particularly rare one, and the resolution for the affected accounts was nuanced since different users were impacted in different ways," Mayer explained. "Some of the affected users were unable to access their accounts, instead seeing an outdated 'scheduled maintenance' page which was a confusing and incorrect message (this has since been corrected and updated). Further, messages sent to those accounts during this time were not delivered, but held in a queue."
The good news here is that there's a big, bright light at the end of the tunnel. Yahoo has been providing ongoing updates on its a specific Help page, the last of which was posted yesterday indicating that 99.9 percent of Yahoo Mail users are now able to access their accounts. Furthermore, the outage message queue is completely cleared and IMAP access is now 100 percent restored.
"We're making progress on restoring full access to messages for affected customers and will update again with more information," the last update reads.
According to data from comScore, as of December 2012 there were 282 million active Yahoo Mail users, compared to Hotmail (now Outlook.com) at 284 million and Gmail at 295 million.