Nested Distribution List Failures

Using Exchange 5.5 SP3, and Outlook 2000 as client. About a week ago all e-mail going to nested distribution lists stopped being received by the recipients. When sending to the DL everything appears normal, message goes from outbox to sent. However, recipients never get the message. Nothing has changed in the way Exchange or Outlook is configured during this time. First does anyone have an idea of how to correct this problem? Second, is it possible for me to trace the path of the e-mail into oblivion in an effort to troubleshoot?