Looks like they are trying to do a reverse resolve on ccsi.com and 
superkill.ccsi.com is refusing connection.  Looks like something 
ccsi.com's spamkiller is doing.

Winship wrote:

> I run several library related professional discussion lists.  I have
> around 100 subscribers at the U.S. Library of Congress.  Since a few weeks
> ago, LC (loc.gov) has been rejecting all of my private mail from
> [log in to unmask] to my subscribers who have written me about list problems.
> I cannot even contact the listowner of several lists at loc.gov, even
> though he thought that he was exempt from all filtering (i.e. blacklist/
> whitelist, etc.); he now knows different.  He also doesn't know what the
> problem is, but thinks it is blacklisting (I don't think he runs the
> email system at LC).
> 
> Could someone take a look at the error message below and give advice as
> to what the problem is, and what might need to be done to correct it?
> I haven't tried [log in to unmask] yet as my experience with "postmaster"
> addresses is that if the item doesn't bounce (no such address) whatever
> I send is studiously ignored, never a reply (wasn't like that 15 years
> ago, but ....).
> 
> I have this problem only with loc.gov, so far.  I can send to anyone at
> LC from my accounts at the U. at Buffalo, but that's a workaround, I need
> to be able to send from ccsi.com.  Any advice appreciated.
> 
> Douglas Winship    [log in to unmask]
> 
> 
> ---------- Forwarded message ----------
> Date: Thu, 29 Dec 2005 14:43:01 -0600 (CST)
> From: Mail Delivery Subsystem <MAILER-DAEMON>
> To: [log in to unmask]
> Subject: Returned mail: see transcript for details
> 
> The original message was received at Wed, 28 Dec 2005 14:38:06 -0600 (CST)
> from infinity.ccsi.com [216.236.168.2] (may be forged)
> 
>    ----- The following addresses had permanent fatal errors -----
> <[log in to unmask]>
>     (reason: 451 4.1.8 Domain of sender address [log in to unmask] does not
> resolve)
> 
>    ----- Transcript of session follows -----
>  ... while talking to sun8.loc.gov.:
> 
>>>>DATA
> 
> <<< 451 4.1.8 Domain of sender address [log in to unmask] does not resolve
> <[log in to unmask]>... Deferred: 451 4.1.8 Domain of sender address
> [log in to unmask] does not resolve
> <<< 503 5.0.0 Need RCPT (recipient)
> Message could not be delivered for 1 day
> Message will be deleted from queue
> 
> 
>