The bogus messages often include pictures of FBI director Robert Mueller, along with the organisation's official seal, letterhead and banner.
"The FBI does not send out emails soliciting personal information from citizens," said the agency.
"The social engineering technique of using the FBI's name is designed to intimidate and convince the recipient that the email is legitimate.
"Please be cautious of any unsolicited email referencing the FBI, Robert Mueller or any other FBI official endorsing any type of internet activity."
The emails are typically a notification of a 'lottery win' or long-lost relative leaving an 'inheritance'.
Other emails offer website monitoring containing malicious attachments, and online auction scams.
The warning comes just six months after the FBI issued a similar alert about spammers using trusted institutions to improve returns.
FBI warns of malicious email scam
By Iain Thomson on Jan 17, 2008 7:33AM