Cisco Cisco Email Security Appliance C160 Mode D'Emploi

The Incoming Relays feature helps your IronPort appliance obtain the IP address 
of an external machine that is sending mail to the IronPort appliance via one or 
more mail exchange/transfer agents (MX or MTA), filtering servers, etc. at the 
edge of the network. In this type of configuration, the IP address of the external 
machine is not automatically known by the IronPort appliance. Instead, mail 
appears to originate from the local MX/MTA (the incoming relay) rather than 
from the external machine. IronPort Anti-Spam and IronPort Intelligent 
Multi-Scan depend on accurate IP addresses for external senders so it is vital for 
the IronPort appliance to have this information.

You should only enable this feature if you have a local MX/MTA relaying mail to 
your IronPort appliance.

 shows a very basic example of an incoming relay. Mail from IP 

address 7.8.9.1 appears to come from IP address 10.2.3.4 because the local 
MX/MTA is relaying mail to the IronPort appliance.