Users are being warned to watch out for a Windows virus posing as a complaint about misdirected e-mail.
The virus claims that e-mail messages are going astray
The variant of the Sober worm tries to trick users into opening an infected e-mail attachment by claiming it contains copies of the wrongly addressed messages.
But the file contains the virus which scours computers for e-mail addresses.
Any harvested address is sent back to the worm's creator and will probably be sold to spammers.
The original Sober virus debuted in October 2003 but since has been regularly updated to appear in a new guise.
The latest variant, like many of the others, is bilingual. The text of the message travelling with the infected attachment is in German when being sent to e-mail addresses ending in .de. All others are in English.
The subject line of the Sober.N variant reads: " I've_got your EMail on my_account!"
The text claims that someone else has been receiving the recipient's e-mail messages by mistake.
It asks the recipient to open up the attached file and review the mis-directed messages to make sure the problem is resolved.
SOBER.N SUBJECT LINES
I've_got your EMail on my_account!
FwD: Ich bin's nochmal
However, the attached file only holds code for the Sober virus.
The broken English the message uses has been seen by some as an attempt to give it authenticity and make people more likely to click on it.
The Sober.N variant was first found on Tuesday 19 April.
Users of Windows 2000, 95, 98, Me, NT, Server 2003 and Windows XP are at risk from the virus.
Security firms urged users to be on their guard and to make sure their anti-virus software was up to date.
The BBC News website has received warnings about Sober.N from Sophos, Blackspider and F-Secure. The number of copies of the virus caught by security firms suggests it is catching out quite a lot of users.
Earlier versions of Sober posed as error messages and claims that a recipient was sending out e-mail infected with a virus.