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Typically, a Directory Harvest attack will target a specific domain with emails to many millions of combinations of email address at that domain, such as: adamsmith@domain.com; adam.smith@domain.com, adam_smith@domain.com, smitha@domain.com etc. Often the domain owner is targeted either for malicious reasons specific to that organisation, or because the business type of the domain owner is incorrectly identified by the attacker. 

One of the best things you can do is removing the catch all account. It is one of the best ways to avoid these types of attacks. If you use an alias correctly these types of attacks only refuse the email. Though it does little to reduce the traffic that might be clogging your mail system it certainly reduces the number of deliverable emails.

These are two good articles. Dark-Traffic  Dark-Mail Rising