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.

Setting Up Site-Wide Preferences for Handling Mail to Nonexistent Users (Mail Bounce)

To customize these options, login to Plesk >> Domains >> Mail >> Preferences

You can choose to:

• Change the default notice if you do not like it,
• Forward all such mail to the desired e-mail address,
• Reject such mail without accepting it and without notifying senders.

  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.