Computer security analysts who fight spam face the same thankless task as goalkeepers: They don't get much credit for the unsolicited email they stop, only demerits for the ones that get through.
But those few messages that wriggle past increasingly sophisticated filters constitute the greatest threats on the Internet.
The sheer volume of spam still threatens to bring the Internet to a crisis point. Up to 90 per cent of all email traffic is spam, a figure that has crept upward in recent years. The forecast isn't good, either.
"We see spam just going up to the point where Internet servers start having difficulty," said Steven Linford, CEO of Spamhaus, a London non-profit organisation that generates a list used by technology companies and organisations running email servers to block spam.
"Spam will increase to where it will be 99 per cent of all email on the Internet," he said. "At that point, governments will start to take notice."
The front line of defence for most computers connected to the Internet is antispam software that tries to determine whether a message is legitimate.
The software can block messages coming from a particular IP address of a computer known to send spam. Messages containing links to potentially harmful websites can be halted using URL filtering. Security vendors can tweak special rules for their antispam engines, such as blocking messages containing certain kinds of text identified as common to spam.
Antispam software usually aims to filter out 98 per cent of bad messages - any higher level of filtering tends to snag real messages and cut off critical business communication.
Hitting the bull's eye
So spammers are aiming for the narrow, 2 per cent window - and even in the last few months have honed new methods to hit the inbox bull's eye, according to experts.
Sophos, one of many security vendors with antispam software, has analysts stationed at four labs - in Abingdon, England; Vancouver; Boston; and Sydney - watching the Internet 24 hours a day, seven days a week for threatening spam and malicious software.
Its lab in Abingdon doesn't look much different from any other office. But it is mission control for security analysts with special rules: no computers or electronic equipment can be brought inside and the room remains locked.
Sophos catches hundreds of new malware and spam samples a day using traps - abandoned email addresses and domains that have been donated for the purpose of spam research.
On a recent day, spam research analyst, Paul Baccas, pulled duty. A message entitled "Let's go" caught his attention after it landed in a spam trap of addresses monitored by Sophos that belonged to a telecommunications company that went bust six years ago.