Using spam filters is one of the most common ways to battle spam. What this means is that the software scans the incoming email, runs it through a series of tests and compares it to known spam criteria, then decides whether it is a good email or a spam message. If it is spam, it will act according to settings.
If it is good, it makes it to your inbox. The perfect spam filter would always get it right, filtering out all spam and letting all valid email through. In real life, however, its a constant battle for accuracy. Filters miss email or falsely flag email all the time. A “false negative” is when the filter does not flag an email as spam when it should have. A “false positive” is when the filter incorrectly flags a legitimate email as spam. For most, a false positive is the worse of the two because perfectly valid email can get removed. This happened to me just the other day when a perfectly valid email contained the word “mortgage” got filtered out. The sender called wondering if I got the email, which of course, I didn’t. The only solution (other than training your filter) is to periodically check your “Deleted Items” folder to see if there are any valid emails in there.