In one embodiment, a method of generating a listing of valid email addresses in a private computer network includes monitoring of inbound emails and outbound delivery failure notification emails. Recipient email addresses of inbound emails may be indicated in the listing as valid email addresses. The delivery failure notification emails may be indicative of receipt in the private computer network of an undeliverable email. The recipient email address of the undeliverable email may be identified in the listing as an invalid email address. Comparing the recipient email addresses of undeliverable emails and inbound emails advantageously allows generation of the listing of valid email addresses in the private computer network without having to ask an email server for such a listing.