Yahoo Japan Corp. said about 2.58 million messages addressed to some 970,000 account holders in its free email service were lost in late August due to a computer system problem.
The company apologized for the inconvenience to users. It said Sunday it will set up an investigation team and map out measures to prevent any recurrence.
Yahoo Mail account holders can check via the service’s website whether emails addressed to them have been lost, along with the addresses of the senders of the lost messages.
Yahoo Japan said it estimates that about 60 percent of the lost messages were unsolicited emails that were supposed to be blocked by the spam filter.
A company spokesman urged affected users not to make contact with unknown email senders.
According to the company, its computer system suffered the glitch around 10:20 a.m. Aug. 28. About 2.6 million account holders were unable to have access to their email accounts until around 8 p.m. that day.
During the period, about 220 million emails reached some 50 million accounts. The lost messages accounted for about 1 percent of the total, company officials said.