SMTP Error 550 Unknown user

We have SBS 2000 using Exchanger Server.  It has been working fine for about 7 months.  2 days ago without any notice in event logs, etc. we stopped getting in external email.  There have been no changes to the server or our ISP service, etc.  I've looked at all the settings, etc  and all seems well.  Any ideas?
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could u give more error message.
this kind of error may come for many reason.
relay reason. may be for stopping spam.
please post more error messages from your server logs.

Error message is in the subject

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sunray, thats why i asked "MORE". this is very generic error message and lead to many  possibilities.
kevincavanaghAuthor Commented:

This is the complete error message I get back when I try to send a message using Hotmail.  You can try from your own email client to see what response you get.  I do not see any errors on the server (I am only looking in the event viewer).  Please let me know what you think...

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

Reporting-MTA: dns;
Received-From-MTA: dns;
Arrival-Date: Thu, 27 Nov 2003 08:41:47 -0800

Final-Recipient: rfc822;
Action: failed
Status: 5.0.0
Diagnostic-Code: smtp;550 unknown user <>

Doesn't the mail server itself create proper logs? Event viewer may not be enough. Rejection due to unknown user should be in the server log.
kevincavanaghAuthor Commented:
Where would I look for this log?
could u see some logs from exchange manager.
START  >  PROGRAMS  >   EXCHANGE  >  then then manager program

also could u printout the full header.
this must be in returned mail when u open a mail. there must be option in view -> option
and there must be internet header, post that header.
Do you have a reverse DNS entry for your mail server?  Some sites are using that to determine who is sending spam and then 550's them if they don't have an entry.
maybe its your own computer administrator that prohibited external e-mails. most companies do this to prevent mail saturation.
peter_fleurOperational Director Commented:
If i test it with a telnet to port 25 everything seems to work. It accepted your name. Maybe its a problem at you isp? Did you receive my mail?


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kevincavanaghAuthor Commented:
Hi all,

the problem with this issue turned out to be with one of the ISP's we use who forwards the email to another ISP we use.  They changed the email mx record somewhere along the line and did not realize it.  This is the first time I am using this website for tech answers and you all have been great.  I am not sure what is referred to as "points".  Do I have to decide how to distribute them?  Please advise...thanks again.
Glad u got it resolved.
yes u have to decide how to distribute points or not distribute and give it the one/two...
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