Zombie PCs accounted for more than half of all spam in July, according to email security firm MX Logic.
About 56 percent of all spam filtered by the company last month was sent from always-on PCs that had been hijacked by malware. Zombie machines were responsible for 62 percent of all spam in June and 55 percent in May.
"These statistics indicate that hijacked PCs have become not only the preferred distribution tool for spammers, but also a primary source of internet pollution," Scott Chasin, MX Logic chief technology officer, said in a statement.
Zombie systems are compromised PCs that can be controlled remotely to send spam or launch denial-of-service attacks.
MX Logic also found that spam-sending domains were the biggest users of emerging email authentication protocols SPF and SenderID.
Nine percent of a sample of 19 million unique emails processed by MX Logic during one week in July were from domains that had published an SPF record, 82 percent of which were spam-sending domains, the company said. Of the 0.15 percent from domains with a published Sender ID record, 82 percent were sending spam.
MX Logic reported a small increase in the amount of email that complied with the federal Can-Spam Act. In a random spam of 40,000 commercial emails, four percent were compliant in July compared with three percent in June.
Earlier this week, Microsoft won a multi-million dollar settlement from reputed spam king Scott Richter.