The two main types of email filters are content filters and blacklists. Content filters analyze the content of email messages for signs of spam or malware. For example, content filters may look for certain keywords or phrases that are commonly used in spam messages, or they may scan for attachments that are known to contain malware. Blacklists maintain a list of known spam senders and IP addresses. When an email message arrives from a sender or IP address that is on the blacklist, the email filter will block the message from being delivered. In addition to these two main types of email filters, there are also a number of other types of email filters available, such as: Whitelists: These filters maintain a list of trusted senders. When an email message arrives from a sender that is on the whitelist, the email filter will deliver the message even if it would otherwise be blocked by other filtering techniques. Bayesian filters: These filters learn to identify spam messages by analyzing the content of messages that have been previously classified as spam or ham (non-spam). Heuristic filters: These filters use a set of rules to identify spam messages. For example, a heuristic filter might look for messages that are sent from a sender with a high spam score, or messages that contain a large number of links.