I'm having trouble emailing a business in Belgium and wondered if anyone may be able to shed light on why this may be.

To quote the warning message I get sent back: (with appropriate sensoring)
"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 48 hours on the queue on telford.theukhost.net.

The message identifier is: a number
The subject of the message is: RE: Hello
The date of the message is: Thu, 3 Mar 2011 12:57:28 -0000

The address to which the message has not yet been delivered is:

[email protected]
Delay reason: SMTP error from remote mail server after RCPT TO:<[email protected]>:
host mx02.mailquest.be [an ip number]: an ip number is not yet authorized to deliver mail from
451-<[email protected]> to <[email protected]>. Please
451 try later.

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."

I have no difficulties emailing everyone else.
I have emailed two people in the same Belgium business and get the same response.
I have checked the email addresses and they are correct.
I have a CPanel Reseller account.
I am sending it using Microsoft Outlook

I want to come accross as a professional business to them but I look a bit naff having to email them from hotmail as a quick fix so will be grateful if anyone has any ideas.