Dictionary attacks are attacks where clients try to send mail to countless potential recipients, whose e-mail addresses are derived from words or names in a dictionary:
email@example.com firstname.lastname@example.org ... email@example.com
If your server doesn't have a list of valid recipient addresses, it must accept these mails regardless whether the recipient actually exists. Then, this onslaught of e-mails needs to be processed as usual (virus check, spam check, local delivery) until, at some stage, the system realizes that the recipient does not even exist!
Then a non-delivery report will be generated and sent back to the sender.
So, for every non-existing recipient, one mail is being accepted and processed, and additionally another ...