I have 3 servers running Exchange 2003 SP2. One of them is not responding to public folder replication requests etc. Further investigation shows that the system email messages to server-IS@domain.com are being NDR'd by the problem server. ADSIedit shows that the address is there for the public folder store under the mail and proxy fields. RUS must be running because new users are getting good email addresses. I created a new public folder tree & store, and system emails to that are failing with NDR 5.7.1 as well. Telnetting using the address fails ( but using the corresponding addresses that the other servers public folder have works). Permissions look alright on the SMTP virtual server as well. All other emails are flowing fine. HELP please Simon