DHA is an attempt to determine the valid e-mail addresses associated with an e-mail server so that they can be added to a SPAM database.

A directory harvest attack can use either of two methods for harvesting valid e-mail addresses. The first method uses a brute force approach to send a message to all possible alphanumeric combinations that could be used for the username part of an e-mail address at the server. The second and more selective method involves sending a message to the most likely user names - for example, for all possible combinations of first initials followed by common surnames. In either case, the e-mail server generally returns a Not found reply message for all messages sent to a nonexistent address, but does not return a message for those sent to valid addresses. The DHA program creates a database of all the e-mail addresses at the server that were not returned during the attack.

This explains how a new e-mail address can start receiving spam within days or hours after its creation.