When somebody sends an e-mail message to an e-mail address that does not exist under your domain, the mail server, by default accepts mail, processes it, and when it finds out that there is no such a recipient under the domain, it returns the mail back to sender with the "this address no longer accepts mail" notice. You can choose to:
This setting can decrease mail server load caused by a large amount of spam, which is often sent to randomly generated user names. However, for spammers, this can somewhat speed up scanning your mail server for valid e-mail addresses.
To configure the site-wide settings for serving mail to nonexistent users: