A team of researchers have become spammers to understand how these cyber criminals work.
The team of computer scientists from University of California, Berkeley and UC, San Diego (UCSD), found that spammers have a very low rate of success, but are still making money.
The team infiltrated the Storm network that uses hijacked home computers as relays for junk mail.
Through this, they found out that spammers only get one successful response for every 12.5m emails they send.
Despite this, massive spam operations, which send out hundreds of millions of emails can turn over millions of pounds in profit every year.
The investigation lasted 1 month in which time seven researchers infiltrated Storm, which was believed to have more than one million machines under its control at one time.
They created their own "proxy bots" and with these took over 75,869 hijacked machines and routed their own fake spam campaigns through them.
The team then ran two types of spamming campaign - one centred upon a legitimate looking pharmacy site while the other mimicked a virus spreading campaign.
"After 26 days, and almost 350 million e-mail messages, only 28 sales resulted", wrote the researchers.
The response rate for this campaign was less than 0.00001%.
"Taken together, these conversions would have resulted in revenues of $2,731.88—a bit over $100 a day for the measurement period", said the researchers.
But for the people who control massive spam operations, this could be as much as $7000 (£4430) a day or $3.5m (£2.21m) per year.