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 protected] [email protected] ... [email protected]
If your server doesn't have a list of valid recipient addresses, then 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 e-mail (the bounce) is generated, and is subject to delivery attempts.
As you can see this course of action wastes precious resources on your servers. Because the server is busy trying to deliver mail it should never have accepted in the first...