A method and system for determining the reputation of a sender for sending desirable communications is provided. The reputation system identifies senders of communications by keys sent along with the communications. The reputation system then may process a communication to determine whether it is a desirable communication. The reputation system then establishes a reputation for the sender of the communication based on the assessment of whether that communication and other communications sent by that sender are desirable. Once the reputation of a sender is established, the reputation system can discard communications from senders with undesired reputations, provide to the recipient communications from senders with desired reputations, and place in a suspect folder communications from senders with an unknown reputation.