Outlook 2007 Send Problems

I have a problem sending to some of my email contacts with Outlook 2007. It's not my equipment as I can contact these persons using outlook 2003. I get the following message.

Your message did not reach some or all of the intended recipients.

Subject: RE: ----
Sent: 08/03/2010 17:34

The following recipient(s) cannot be reached:

-------- on 08/03/2010 17:34
550 5.7.1 <>... Relaying denied. IP name lookup failed [188.116.209.12]

Has anyone seen this before? How did you fix it?

Thanks! :)

I would look into changing the default sending account, especially if you have multiple email accounts in Outlook.

Is there anything you can identify that differentiates addresses that work from those that don't? Maybe certain domains?

rDNS issues... Are you in a domain environment? Is this email address for work?

My email address is a normal pop3. I only use one email account.

The people I am trying to send to are in a large company with secure servers, all one domain. I cannot contact anyone with my company email addresses, but all others in my contacts are okay.

The only difference I have is that I got a new notebook with Windows 2007 and office 2007 professional. I'm using the same internet provider and can connect either by LAN or wireless. My old notebook uses office 2003 & Windows XP. Everything works with the old notebook, just the new one doesn't.

Confused!!!