A recipient computer system may receive one or more electronic messages. The recipient computer system may send one, two, or more challenge messages to the sender of the electronic messages in determining whether to deliver the electronic message. The recipient computer system may accept or reject an electronic message based on the response, or lack of response, to the challenge messages. The recipient computer system may send two or more challenge messages at intervals. A sender computer system may record activity of the sender computer system for a recipient computer system to use in determining whether to deliver an electronic message. A sender computer system may provide a summary of that activity with an electronic message. A sender computer system may provide an authorization key for a recipient computer system to use in determining whether to deliver an electronic message. The authorization key may be generated by a third party.