DENVER -- A Colorado man accused of sending hundreds of thousands of spam e-mails has been sentenced to 21 months in federal prison after pleading guilty to tax evasion and falsifying e-mail headers.

Edward "Eddie" Davidson, 35, of Louisville, was also ordered to pay nearly $715,000 to the Internal Revenue Service. He was sentenced Monday and ordered to report to prison authorities in May.

Federal prosecutors say Davidson's operation used false e-mail headers to disguise the sender. Prosecutors say some of the spam was meant to dupe stock investors and manipulate markets.

Authorities say Davidson made at least $3.5 million sending e-mails for nearly 20 companies.

His business, Power Promoters, operated from July 2002 through April 2007. The primary nature of Davidson's business was to promote companies by sending spam, the U.S. Attorney's office said. During 2002 through the middle of 2005, Davidson's spamming activities were provided on behalf of companies to promote watches, perfume, and other items, authorities said.

But in the middle of 2005 through 2006, Davidson sent spam on behalf of a Texas company for purposes of promoting the sale of the company's stock. The company generated its income through selling stock (commonly referred to as "penny stock") on behalf of small companies on the public market.

Davidson, aided by several sub-spammers, sent hundreds of thousands of unsolicited e-mail messages to potential purchasers throughout the United States and the world, with messages touting the penny stock as an excellent investment. He operated his spamming activities from his home in Bennett, Colo., where he had a large network of computers and servers, which facilitated his business, the indictment said.

"Spammers who abuse the Internet to manipulate the stock market may find themselves in federal prison," said Troy A. Eid, United States Attorney for the District of Colorado.