Check why mail fails to deliver.

I have a client: "[login to view URL]" My client has a contact: "[login to view URL]" For some reason mails between these two contacts fails to be delivered. My client has no problems with any other mail. The other party claims also not to have any problems with other mail. I have confirmed that the other party recieves mails from yahoo: no delivery failure AND i got a response. Checking DNS settings clearly states to me that the error is at the other party: [login to view URL] [login to view URL] I pay 10 euro milestone in advance and I will grant it to the one that makes me understand the problem in a way I can either fix the problem OR can clearify why Yahoo does work. PLEASE write your first 2 WORDS in capitals, so I know you understand.