Spam filters are used to prevent spam. Mail filters check the Internet address( URL) and scan the content of the e-mails. Emails that contain certain words or topics, or that come from certain servers, are discarded by the spam filter, also known as a Bozo filter.
Since spammers use invalid IP addresses, spam filters use reverse DNS( rDNS) to convert the incoming IP address into a domain name. If no valid domain name is found in the DNS system, then the spam filter blocks the message. As spammers continue to develop new techniques, spam filter technology must rise to these challenges. For sufficient spam protection, besides the mentioned technique, there are custom filters, reputation filters, file attachment filters, URL filters and heuristic filters. In the case of the user-defined mail filter, the user himself creates his own positive and negative address lists of customers.
The reputation lists are created by official institutions. The customer can retrieve these lists and use them for his spam filter. The lists are constantly updated. File attachment filters block only attachments with specific characteristics. URL filters also work with lists. The mail server recognizes whether the Internet address is registered when the e-mails are received. Heuristic mail filters analyze the structure of the incoming e-mail. In doing so, the filter checks components such as the subject line or the content.
DNS spoofing uses an additional entry in the Domain Name System (DNS) to detect whether the sending server matches the sender's address. Mails with fake senders that can be assumed to be spam are not delivered.