An email filter is a piece of software that’s installed on a POP3/IMAP server and ‘scans’ all incoming messages so as to impede any unrequested ones from entering a given inbox. A few examples of such email messages would be: offers for pills or money, fake banking notices or attachments that contain malicious software sent with the idea to infect your computer. Spam filters usually check the content of an email and when they spot some keywords or other dubious content, they either erase the email or re-send it to the Junk/Spam folder instead of the Inbox folder. Certain companies mix their own spam filters with up-to-the-minute databases from spam-tracking organizations, so as to guarantee better security for their customers. Such databases contain patterns, mail server IPs and other info about spam emails recently uncovered by these organizations.