A DHA is one method used by spammers to collect valid e-mail addresses. Spammers either target these addresses directly in their own spam attack, or to sell them on to other spammers.

The spammer first selects a domain (let’s say ‘victim_domain.com’) and then sends speculative e-mail messages to possible addresses within that domain (for example, ‘jack@victim_domain.com’, ‘jill@victim_domain.com’, etc.). If the e-mail server at ‘victim_domain.com’ doesn’t reject the e-mail, the spammer knows that a given e-mail address is valid and can be used as a target in a spam attack.