Microsoft's Hotmail, a free, Web-based email service, said that the problem was only a matter of appearances, and that no mail had been lost.
"We discovered an issue that caused some email header information to appear unavailable," said Hotmail product manager Deanna Meyer. "This has already been resolved, and we are currently updating all Hotmail servers."
Meyer did not say how many users' headers disappeared. She did say that affected users should see their email headers reappear in the next few days as Hotmail continues updating its servers.
The disappearances alarmed Hotmail users, who suspected that Hotmail had erased their old messages.
"All the mail I had previous to yesterday is gone," complained one user in an email to CNET News.com. Later today, that user said his email had been restored.
Another wrote, "Hotmail wiped my in-box, which last night contained 50-plus messages and this morning was empty." As of this afternoon, the user said his email was still missing.