Take a look at the headers. Who is the reporting MTA (the MTA that reports the user doesn't exist on the server?) _____ From: Gene O'Brien [mailto:Gene@xxxxxxxxxx] Sent: Tuesday, December 16, 2003 9:41 AM To: [ExchangeList] Subject: [exchangelist] Non Deliverable Problem http://www.MSExchange.org/ Hi All, We're using Exchange 5.5 on Windows 2000 server. I have one user who consistently has a problem that I have had trouble tracking down. When he writes an email to someone outside our system on the internet, it gets through just fine. However, if he RESPONDS to an email that someone has written, about 1 out of 10 times he gets the message below: -----Original Message----- From: System Administrator Sent: Wednesday, December 10, 2003 12:02 PM To: Healey, Thomas J. Subject: Undeliverable: Timber follow up Your message did not reach some or all of the intended recipients. Subject: RE: Timber follow up Sent: 12/10/2003 12:02 PM The following recipient(s) could not be reached: Healey, Thomas J. on 12/10/2003 12:02 PM 551 5.7.1 no such user on our server However, if he originates a NEW mail to that person, it gets through just fine. At first I thought maybe it was a problem with AOL, as the few examples he originally gave me were AOL users - so I figured they had mail controls enabled that were just blocking his mail. But it's not just AOL addresses. Any ideas on what to look at for this? I've got a very frustrated user and I'm not sure where to look for this. Thanks! Gene ------------------------------------------------------ List Archives: http://www.webelists.com/cgi/lyris.pl?enter=exchangelist Exchange Newsletters: http://www.msexchange.org/pages/newsletter.asp Exchange FAQ: http://www.msexchange.org/pages/larticle.asp?type=FAQ ------------------------------------------------------ Other Internet Software Marketing Sites: Leading Network Software Directory: http://www.serverfiles.com No.1 ISA Server Resource Site: http://www.isaserver.org Windows Security Resource Site: http://www.windowsecurity.com/ Network Security Library: http://www.secinf.net/ Windows 2000/NT Fax Solutions: http://www.ntfaxfaq.com ------------------------------------------------------