A method and system for determining whether an IP address is part of a bot-network are provided. The IP-address-aggregate associated with the IP address of an e-mail sender is determined. The IP-address-aggregate is associated with an IP-address-aggregate-category based on the current SMTP traffic characteristics of the IP-address-aggregate and the known SMTP traffic characteristics of an IP-address-aggregate-category. A bot-likelihood score of the IP-address-aggregate-category is then associated with IP-address-aggregate. IP-address-aggregate-categories can be established based on historical SMTP traffic characteristics of the IP-address-aggregates. The IP-address-aggregates are grouped based on SMTP characteristics, and the IP-address-aggregate-categories are defined based on a selection of IP-address-aggregates with similar SMTP traffic characteristics that are diagnostic of spam bots vs. non-botnet-controllers spammers. Bot likelihood scores are determined for the resulting IP-address-aggregate-categories based on historically known bot IP addresses.