Question : Inbound Email not received and no ndr

I am running Exch Server 6.5 SP2 on on SBS2003 SP2. Three days ago inbound email stopped and senders do not get a ndr. An external check of our MX record returns our public IP address OK.

Answer : Inbound Email not received and no ndr

Has anonymous access been disabled from the SMTP Virtual Server?  With this disabled - no-one will be able to get any mail in to your server.