When your server has become a haven for spammers, it is never a pleasant ordeal. Your server will probably be blacklisted, causing many of the emails you send to bounce back, and you may have serious connection problems due to the spammer using valuable system resources.
There are a couple of ways in which spammers will use a server to send their emails. The first is through an open relay in your mail server. This is something that you can easily fix in both Postfix and Sendmail. I will post more about fixing an open relay tomorrow. The second method is by signing up for a legitimate hosting account with your web hosting service, but then using the account to send spam. By the time you figure out what they have done, they are usually long gone.
The third and probably most nefarious method of sending spam from your server is through an actual attack on your server. When a hacker finds a hole in your security, they will install a bot on the server. That bot will then act as a small mail server for the hacker, all without your knowledge. You may not even notice it unless it causes some side effects. All three methods should be fixed with increased security and careful scanning of your server for possible threats.
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