September was back-to-work month for junk mailers, with one web monitoring firm counting a monthly total of seven billion spam, phishing and virus infected emails emanating from 50,000 separate computers at any given time.

Email monitoring firm Postini reported that 80 per cent of all the email it tracked was spam and that it blocked 22 million viruses in a single month.

"At any given moment in September, Postini was tracking 50,000 computers that were exhibiting signs of malicious behaviour,” said Scott Petry, founder, chief technical officer and executive vice president of product development at Postini.

The firm said hundreds of thousands of separate web-connected computers showed malicious behaviour, and that on average, each attacked in two-hour bursts before becoming dormant again.

The global nature of the nuisance was revealed with cities as far apart as Seoul, South Korea and Herndon, Virginia in the US reporting high levels of malicious activity.

Postini's September top five viruses were:
1. netsky 4,078,646
2. mytob 2,880,131
3. bagle variants 2,541,480
4. mime 2,427,418
5. mydoom 1,495,505