Those mailbox-names are a bit peculiar, but my first guess would be SPAM and/or virus/trojan traffic where your 'XXXXXX' list was forged in the From field and the addresses shown in your DEMR were in the To field.  However, showing "no such list" as the error condition discredits that idea somewhat - usually such DEMR entries (at least when seen on the site I administer) are the result of a remote mailer sending a non-delivery-notice after accepting the spam/malware message - this looks like your site was actually trying to deliver a message to the CUNY machine and got a reject in the SMTP conversation.

You might try a search of your site LISTSERV log files, and the logs on the outbound-mail-handler system your LISTSERV delivers through, to glean a bit more detail about what is happening.

>>> "Helmke,Richard A" <[log in to unmask]> 11/19/08 8:58 AM >>>
We are running LISTSERV 15.5 and lately I have noticed on a number of lists a "550 No such list." error.  One thing in common is the use of a HEX name, like 2.0006194D at a LISTSERV address (see example below). Since we aren't sending email to these addresses, where do they come from and is there an easy way to block them without disrupting "normal" LISTSERV "network" activities?
-Rich
-- 
Richard A. Helmke                Internet: [log in to unmask] 
Assoc Prof, Computer Science              [log in to unmask] 
System Manager, Technical Services              Voice: 708-209-3221
Concordia Administrative Information Systems    Fax:   708-209-3177
River Forest, IL 60305-1499

-----Original Message-----
From: CAIS/CUS LISTSERV Server (15.5)
[mailto:[log in to unmask]] 
Sent: Wednesday, November 19, 2008 12:01 AM
To: POSTMASTER
Subject: XXXXXX: Daily error monitoring report

The following X subscribers are currently being monitored:

Err First Last  Address
--- ----- ----- -------

  1 11/05 11/05 [log in to unmask] 
                Last error: 5.1.1 Mailer [127.0.0.1] said: "550 No such list."

  1 11/09 11/09 [log in to unmask] 
                Last error: 5.1.1 Mailer [127.0.0.1] said: "550 No such list."

  1 11/18 11/18 [log in to unmask] 
                Last error: 5.1.1 Mailer [127.0.0.1] said: "550 No such list."

  1 11/09 11/09 [log in to unmask] 
                Last error: 5.1.1 Mailer [127.0.0.1] said: "550 No such list."