Spammers are making profit even though they are only getting one response for every 12.5 million e-mails they send, researchers who hacked into the infamous Storm spam network have found.
The spam study was carried out by computer scientists from University of California Berkeley and University of California San Diego.
For the month-long study the computer scientists infiltrated the Storm network, which uses hijacked home computers as relays for junk mail.
At its height, Storm is believed to have had more than one million machines under its control.
The researchers created several "proxy bots" on the Storm network to send out their own spam.
The team used these machines to control a total of 75,869 hijacked machines on the network and routed their own fake spam campaigns through them.
Linked to the spam e-mails they sent out, the team created a legitimate looking pharmacy site selling "male enhancement herbal remedies".
The fake pharmacy site was made to resemble those run by Storm's real owners, but it always returned an error message when potential buyers clicked a button to submit their credit card details.
The researchers sent about 350 million email messages over 26 days, which resulted in only 28 sales.
The response rate for the campaign was less than 0.00001%. Direct mail campaigns aim for 2.15%.
Even though the experiment saw a poor conversion rate, the researchers said that by sending out many more messages, the owners of the Storm network could be earning £4,500 a day, or £1.6m a year.