Spam is on the rise once more, thanks in part to a new technique.  Unsolicited email accounted for 90.4 percent of all messages received on corporate networks during April, an increase of 5.1 percent from the previous month according to Symantec's MessageLabs Intelligence unit.

Adam O'Donnell, a researcher at Cloudmark, a provider of anti-spam tools, noted that in addition to using botnets, spammers in recent months have been experimenting with a new way to sneak unwanted email past corporate filters.

Often, he said, a spammer will rent legitimate network services, often in an Eastern European country, and then blast a large amount of spam at the network of a specific ISP. The idea is to push as many messages as possible onto the network before any kind of filtering software detects the incident. O'Donnell estimates that hundreds of thousands of such messages are sent each day without detection.

Social networks are also becoming an increasingly important tool for spammers.

Security experts note that social-networking spam can't be filtered at the corporate firewall and appears to come from friends of the recipients.