MSN admins Mails disappears into ether

Is there any MSN admins on the list that can help. We have a customer that sends mail to an msn account and sometimes it goes and sometimes it does not and they are getting very frustrated with our service of course blaming us. I have logs that show we sent the mail and it was accepted for delivery but the person on the other end never gets it. This customer can send the same mail 5 minutes apart for testing and we show it being delivered and accepted both times but the guy on the other end will get 1 out of the 2 messages so we do not think it is being marked as spam on your end and dropped. If anyone can please contact me off list or if anyone has had any success with contacting a MSN admin that info would be greatly appreciated.
Thank you