Mail Queue shows Recipients as valid user id @ random domain

[JeffreyZ]

I've been seeing something strange in my mail queue in the past week. The Recipient email address has a correct user ID but uses a random domain (unrelated to my hosting). These show as "undeliverable mail return to sender". Out of 84 emails in queue, about 60 are this type of thing:
==========
Received: by my-hosting.net (Postfix)
id F2334J76CE; Thu, 10 Apr 2014 16:30:58 -0400 (EDT)
Date: Thu, 10 Apr 2014 16:30:58 -0400 (EDT)
From: MAILER-DAEMON@my-hosting.net (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: a-unique-user-id-in-plesk@comcastbusiness.net
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
boundary="A11ABC3452.1397161858/my-hosting.net"
Message-Id: <20140410202058.F2CABC76CE@my-hosting.net>
==========
Is this a hack attempt and if so, is there anything to be done about it?

 

[InderS]

Hello,

I can see you are getting the all these bounce back massages which was sent from your server. I will suggest you please monitor your server mail queue/logs and try to find out which user is sending these mails from your server.