Is it possible that someone(s) else using the same ISP have eroded it's 
senderbase to the point where mail gets rejected based on that or some 
spam database?

What MTA are you using on your end?

What do the full headers look like?

Any connection logs (postfix/sendmail/whatever)?

Any Barracuda/Ironport (or similar) appliances or relays in the chain?

-C
[log in to unmask] wrote:
> One of the subscribers to the list [log in to unmask] has sent me a copy of a message he has received from his ISP, ACSWorld.com::
> 
> ---- Original Message -----
> From: "Postmaster" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Monday, December 11, 2006 11:00 AM
> Subject: Undeliverable Mail
> 
>> Delivery failed 97 attempts: [log in to unmask]
>>
>> Unexpected connection response from server:
> 
> My subscriber has tried several times to resend his message, but it doesn���t go through.
> 
> Has anyone any advice for me?  He has copied simultaneously to me his attempts to resend his message, and I cannot see anything wrong with them.  He seems to be reluctant to discuss the problem with his ISP.
> 
> Thanks for your help.
> 
> Harry Dewey, listowner, [log in to unmask]

-- 
Christopher Wilson
Information Systems Coordinator
ISS Enterprise Systems
The George Washington University
[log in to unmask]