In the mail logs, you might see the error for "envelope sender domain does not exist."
The Cisco Email Security Appliance (ESA) makes an MX record query for the domain of the sender address. It then performs an A record lookup on the MX records. If, for any of these queries, the DNS server returns 'NXDOMAIN' (there is no record for this domain), the ESA treats that domain as non-existent. This falls into the category of "envelope senders whose domain does not exist." NXDOMAIN can mean that the root name servers are not providing any authoritative name servers for this domain.
Mon Nov 20 10:50:05 2006 Info: New SMTP ICID 1605269 interface InternalNet (10.101.150.43) address 192.168.23.36 reverse DNS host small-mail.example.com verified yes Mon Nov 20 10:50:05 2006 Info: ICID 1605269 ACCEPT SG UNKNOWNLIST match sbrs[0.0:10.0] SBRS 3.5 Mon Nov 20 10:50:05 2006 Info: ICID 1605269 Address: < email@example.com> sender rejected, envelope sender domain does not exist Mon Nov 20 10:50:05 2006 Info: ICID 1605269 lost Mon Nov 20 10:50:05 2006 Info: ICID 1605269 close