Sophos senior technology consultant Graham Cluley said that if Hotmail users are inexplicably unable to log into their account, then it is possible their email account has fallen victim to this attack.

"Hackers aren't just interested in breaking into email accounts out of curiosity or because they want to read your spam," . "They're also interested in stealing your identity and perhaps using an email account hack as a method to crowbar their way into other online accounts under your control."

Last year, the webmail accounts of Google, Yahoo and Hotmail users were hit by a series of . Hackers were able to gain access to webmail accounts and send specially crafted phishing email messages to several thousand victims.