The aim of this diploma thesis is to design a method for outgoing spam detection without the possibility of analyzing the user content of electronic mail messages (e-mail). This problem mainly concerns ISPs (Internet Service Providers) who are blacklisted for such traffic. Such a type of spam traffic detection may be beneficial for ISPs to alert end users before they are listed. For the detection, it can be considered to use MX records in the Domain Name System (DNS) in combination with the detection of unusual traffic. In the theoretical part of the thesis, describe the possibilities of spam detection from monitoring network flows. In the practical part of the thesis, propose a method for spam detection from monitoring network flows without knowledge of user data and implement it in the chosen programming language. Python programming language is preferred.