Some messages appeared stripped of their headers, showing up in the in-box with the subject "(none)" and lacking any information about sender, recipient or subject when the message was opened. Those messages, however, did contain the body of the email text.
Other messages weren't as lucky, showing up devoid of any content or labeling.
Yahoo said the problem arose out of efforts to restore the flow of information between its various services and applications following the denial of service attack.
In a denial of service attack, Web sites succumb to heavy bombardment of bogus requests for information. When the targeted server responds, the attackers' system steps up the barrage by sending more requests. The affected Web site struggles to keep up with the mounting number of requests, slowing performance for users or ultimately crashing the system.
The assault on Yahoo was followed today by attacks on Buy.com and eBay.
A bug accidentally introduced during Yahoo's denial of service cleanup caused some Yahoo Mail messages to become garbled, according to the company.
But a company representative said no information was actually lost, and Yahoo engineers are at work restoring headers and bodies to those email messages. Users who deleted those mysterious messages should retrieve them from the Trash folder pending the restoration of data, the representative said.
Users who have deleted blank messages and subsequently emptied their trash appear to have lost their email for good.
Yahoo would not estimate when the fix would be complete.