TECHNIQUES FOR DEFENDING AN EMAIL SYSTEM AGAINST MALICIOUS SOURCES

In one embodiment, a server computer determines whether an email entering a private computer network is malicious (e.g., part of a directory harvest attack or bounce-source attack) by determining the recipient email address of the email and the Internet Protocol (IP) address of the source of the email. When the server computer determines that the email is malicious, the server computer may reject the email by sending a non-deterministic response to the source of the email. The non-deterministic response may include an error message that is different from the actual reason why the email is being rejected. The rejection may be sent as an immediate reply or postponed, for example.