Cisco Cisco Email Security Appliance X1050 Betriebsanweisung

 describes the SMTP Call-Ahead Server Profile advanced settings:

The interface used to initiate the SMTP conversation with 
the SMTP server. 

Choose to use the Management interface or Auto. When 
you select Auto, the Email Security appliance attempts to 
automatically detect an interface to use. The Cisco IronPort 
interface attempts to connect to the SMTP server in the 
following ways:

If the call-ahead server is on the same subnet as one of 
the configured interfaces, then the connection is 
initiated by the matching interface.

Any configured SMTP routes are used to route the 
query.

Otherwise, the interface that is on the same subnet as 
the default gateway are used. 

The MAIL FROM: address to be used for the SMTP 
conversation with the SMTP server. 

The number of seconds to wait for a result from the SMTP 
server. This timeout value is for a single recipient 
validation request which may involve contacting multiple 
call-ahead servers. See 

.

The action to be taken when a recipient validation request 
fails (due to a timeout, server failure, network issue, or 
unknown response). You can configure how you want the 
Email Security appliance to handle the different responses. 
See 

.

The action to be taken when a recipient validation request 
temporarily fails (and a 4xx response is returned from the 
remote SMTP server). This can occur when the mailbox is 
full, the mailbox is not available, or the service is not 
available).

See 

.