Outbound Mail Lost??

Client network is Windows XP Pro to Windows NT server 4.0 SP6a+. One client, running Outlook 2000 is unable to get mail thru to SOME of his contacts, other outbound internet mail goes thru fine. We’re using a software mailserver, Winproxy, and tracking the outbound messages yields “messages sent successfully”, but the recipient never receives the email. ISP tested with me this a.m. and says the email never arrived at their server. Unknown if this has been going on only since the XP install or not. Anyone else seen this problem?? Clients now worry (understandably) that they are not receiving all of their mail. THANKS!