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Exchange Server is rejecting email from all senders:  Error smtp;553 Relaying is not supported

I am running an Exchange Server SP2 single mail server.  We can send email but all clients that try to send us emails are getting messages back saying"

 "Final-Recipient: rfc822;employee1@Retail.com
Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;553 Relaying is not supported
X-Display-Name: employee1@Retail.com"

The only recent change is that we installed Trend Micro Worry Free Business Security Advanced about a week ago but the issue just started yesterday monring.  It seems like our mail server is rejecting recipients because it thinks the email is being relayed but every single email is being rejected, even from domains that we know are not relaying email.  ESM Message Tracking does not show me what is happening when the senders attempt to send emails.  It seems like it is not even reaching the Exchange server at all.  Please help.
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You should check your receive connector and see if "anonymous" is checked in EMC
Neil RussellTechnical Development Lead

Commented:
Can you post a FULL 553 message. It would seem that that is NOT from your exchange server at all. I would talk to your supplier of Trend Micro Worry free, or was this a self setup job?
Commented:
Thank you guys.  I actually figured it out.  The MX Record was pointing to a server that was handling their spam filtering.  They recently cancelled their subscription to this service which resulted in mail being sent to their domain to be undeliverable.  I contacted their DNS host and had them modify the MX Record to point to their Exchange server and now mail has started flowing again.

Author

Commented:
I have resolved the issue myself.