Email misery

[Spike]

Here's a good one for all you IT experts. I have a new client that I've been trying to send emails to. I receive emails from him no problem, but all emails from me to him don't get through. His IT department have checked for any blocks at his end and I've checked with my email service provider for any possible blocks at their end. Neither can see any reason why my emails should not get through. Here's a sample of an error message I get back when sending an email to this client:
QUOTE
This message was created automatically by mail delivery software.
A message that you sent has not yet been delivered to one or more of its recipients after more than 24 hours on the queue on cp32.myhostcenter.com.
The message identifier is: 1MsgOy-0005oe-RN
The address to which the message has not yet been delivered is:
fred.client@customer.co.id
No action is required on your part. Delivery attempts will continue for some time, and this warning may be repeated at intervals if the message remains undelivered. Eventually the mail delivery software will give up, and when that happens, the message will be returned to you.
No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 8.5.409 / Virus Database: 270.14.0/2406 - Release Date: 09/30/09 18:56:00
UNQUOTE

I'm using Vista with Outlook2003 as the email client. I'm not having any problems communicating with any other email addresses.

Any ideas?

[Jedi]

Have you tried sending from a different email client, eg. gmail or hotmail?

[Spike]

Have you tried sending from a different email client, eg. gmail or hotmail?

Yes, tried sending from my Hotmail address and waiting to see if clients receives it. What's your thinking?

[Jedi]

Well that would clearly isolate it as their issue.