Selected mail not delivered

This is on Exchange 2007. I have a group of users that are unable to send emails to a certain domain from within Outlook. However if they use OWA then the emails are delivered to that domain without problem. All email originating from the problem domain and sent to us are delivered without problem. Both businesses are my clients so I have access to both Exchange servers for troubleshooting. I have tried whitelisting our domain on the target domain's exchange server but that has not helped. There has never been any non-delivery errors generated at either end. Also we have a second group of users on our exchange server. They have a different reply to domain. Those messages are being delivered to the problematic domain without any problems.

I am completely at a loss to explain why a subset of our users would be unable to send to a domain from their desktop while at the same time have the ability to send from OWA. I had originally thought it was a filtering issue based on the different reply to domains but whitelisting the domain did not help and as I said they users are able to send using OWA.

Anyone have any suggestions on how to troubleshoot this? I am not even sure which server I should be looking at for the problem.