Just as spam fighters thought they were getting the upper hand in the battle against unsolicited email (New Scientist, 5 February, p 26), the spammers struck back this week. Their potentially devastating new trick is an update to the software they use to remotely control hacked "zombie" computers, and will make spam emails far tougher to block.

Zombies are domestic PCs that spammers have converted into email servers using a virus. Anti-spam firms compile lists of zombie computers so they can block mail from them. The new software gets round this by routing spam through the zombie PC's internet service provider instead of sending it direct. This makes it harder to block as ISPs relay vast amounts of legitimate mail.