Technologies are described herein for managing the IP addresses of outbound e-mail messages based on sender reputation. A range of reputation scores is divided into a number of reputation segments, each covering a span of the reputation scores. The available IP addresses are distributed evenly among the reputation segments. The span of reputation scores for each of the reputation segments may be periodically adjusted based on historical data regarding a volume of e-mail messages sent by sender reputation score. In order to select a source IP address for a particular outbound e-mail message, the reputation segment encompassing a sender reputation score regarding the sender of the e-mail message is determined. An IP address is selected from the IP addresses assigned to the reputation segment and returned to be utilized as the source IP address in sending the outbound e-mail message.