1. A method performed by a computing device, the method comprising: receiving, from a plurality of advertiser devices, a plurality of advertisements, for each advertisement, at least one corresponding personal information value, and for each advertisement, a corresponding price; granting, to an email device, access to a first email account, in which the first email account has a user and has an account balance; calculating, for each of a plurality of personal information types, a corresponding price; instructing the email device to display the plurality of personal information types and the corresponding prices; instructing the email device to display an input control that permits entry of a minimum price, and an input control that permits entry of a respective personal information value for each of a second plurality of personal information types; receiving, from the email device, a first set of personal information values of the user; receiving, from the email device, a minimum price; determining a first subset of the advertisements that match the first set of personal information values, without making the first set of personal information values available to any of the plurality of advertiser devices, in which each advertisement of the first subset matches at least one of the first set of personal information values, and each advertisement of the first subset has a corresponding price which is not less than the minimum price; sending, to the first email account, a plurality of email messages, in which each email message includes a respective advertisement of the subset of advertisements; determining an amount to pay the user based on the prices corresponding to the subset of advertisements; increasing the account balance by the amount to pay the user; after sending the plurality of email messages, receiving a command to withdraw at least one personal information value of the first set of personal information values; determining a second subset of the advertisements that match a second set of personal information values, in which the second set of personal information values includes each of the first set of personal information values except for the at least one personal information value, whereby the second set of personal information values does not include the at least one personal information value; and sending, to the first email account, a plurality of email messages, in which each email message includes an advertisement of the second subset of advertisements.