If you find an e-mail from Bill Gates in your inbox, the chances are that the message is a computer virus.
The subject line says the e-mail is from email@example.com
Security experts are warning that a mass-mailing worm is spreading widely across the internet, sometimes posing as an e-mail from the Microsoft boss.
The Windows virus, called Sobig-C, forwards itself to any addresses found on the infected computer, using several faked addresses such as firstname.lastname@example.org.
This is the second time in recent weeks that virus writers have used messages pretending to be from Microsoft to lure unsuspecting users into opening a malicious program.
Spate of attacks
Anti-virus companies have rated Sobig-C as a high risk virus. According to e-mail filtering firm MessageLabs, it was first spotted on 31 May in the US.
It has now spread across the world, with the UK bearing the brunt of the rogue e-mails.
"Sobig-C is the latest in a recent spate of mass mailers and is proof that the problem of virus outbreaks has not gone away," said Paul Wood, Chief Information Analyst at MessageLabs.
Virus in May also pretended to be from Microsoft
"We have seen over 15 new viruses in the past month, and recently stopped five new variants in one day."
People have been warned to be on the lookout for e-mails containing subject lines such as "Re: Movie", "Re: Approved", or "Re: Your application", with the message "Please see the attached file".
The worm uses a number of different attachment names including "screensaver.scr", "movie.pif" and "documents.pif".
Like other mass-mailing worms, Sobig-C does not harm a computer but will forward itself to others.
It spreads through e-mail addresses, plundered from the Outlook address book as well as from other networked computers.
In a sneaky trick, it will make changes to a computer so that the machine will be re-infected when you restart it.
People are advised to delete any suspect e-mails straight away and to update their anti-virus software to stop the Sobig-C virus.
Virus writers are becoming ever more cunning in how they persuade people to open infected messages.
Tennis stars, the war on Iraq and even the Sars virus have been used in the past to lure net users.
But recently virus writers have turned to take advantage of the name of the world's largest software company.
A couple of weeks ago, a virus masquerading as a message from Microsoft support was spotted in at least 69 countries.