Crime sometimes pays
Most spam is simply advertising. However, a small proportion of messages have a malicious purpose, which can range from simple vandalism through to theft and industrial espionage.
For example, a recent virus made infected PCs send messages to addresses in users' address books. Because the text did not form recognisable words, and because the originator of the emails appeared to be valid users, these mails were not filtered by many types of anti-spam engine.
Infected firms found ordinary emails were delayed because outgoing spam was consuming bandwidth. Consequently some users were unable to work normally, and IT staff spent many hours managing queues on overworked mail servers and antivirus gateways. Rather than using a filter, it seems a bandwidth-limiter that stops users from sending more than a few emails each minute is the only way to prevent such mails clogging the internet. But though this type of vandalism has serious consequences, for many firms a risk analysis would probably show the cost of running an anti-spam gateway is higher than the costs of such incidents.
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