Spam is increasingly a core problem affecting network security and performance. Indeed, it has been estimated that 80% of all email messages are spam. Content-based filters are a commonly deployed countermeasure, but the current research focus is now moving towards the early detection of spamming hosts. This paper investigates if spammers can be detected at the network level, based on just flow data. This problem is challenging, since no information about the content of the email message is available. In this paper we propose a spam detection algorithm, which is able to discriminate between benign and malicious hosts with 92% accuracy.