Web giant MSN is facing a barrage of complaints from Hotmail users who claim that some emails have been deleted from their accounts.
A number of users resorted to forums to vent out their frustration after they logged into their accounts to find empty inboxes and missing folders.
It is unclear whether the problem lies with Microsoft, which owns MSN, or whether it is the work of hackers.
Some Microsoft Hotmail users are starting off the New Year scrambling to get back old emails, the Daily Mail reports.
A chorus of frantic users posted complaints on Microsoft's online forum that all their messages had disappeared.
Others complained that the majority of the emails in their inboxes were sent to their deleted mail folders instead. It is unclear from the posts how widespread the problem is.
The free web-based email service is the world's most used with about 360 million users, according to comScore Inc. It is not yet known how many account holders were affected.
Windows Live support technicians said in numerous threads that the Hotmail team was aware of the problem and was working on it.
"At this point it appears to be a limited issue, and Microsoft is working with individual users who are impacted. We apologise for any inconvenience to our customers," Microsoft spokeswoman Catherine Brooker said.
She declined to disclose what caused the glitch.
Microsoft's forum contains 476 pages of complaints about lost and deleted e-mails that date back to early November last year.