This is the major form of spam that is running rampant over the internet. Often called 'BotNets' or 'SpamBots', hackers and spammers exploit vulnerable operating systems with viruses or programs, often hidden in images or web pages on the internet. Often the only thing you notice is that your computer runs a little slower. One profitable way is using a 'BotNet' to get the computers to pretend to be email servers, and send out millions of spam messages.
There are some differences between your computer and a real email server, and MagicSpam can detect that. Anti-spam companies and ISP's use spamtraps to detect compromised home computers and provide the IP's they use for home style connections. This information is used to create a databases of IP addresses that can be compromised to send spam. Some ISP's can block over 95% of inbound attacks using one or more of these lists.