Cisco Cisco Email Security Appliance C160 Mode D'Emploi

By rejecting all hosts other than the ones you specify, the 

listenerconfig

 and 

systemsetup

 commands 

prevent you from unintentionally configuring your system as an “open relay.” An open relay (sometimes 
called an “insecure relay” or a “third party” relay) is an SMTP email server that allows third-party relay 
of email messages. By processing email that is neither for nor from a local user, an open relay makes it 
possible for an unscrupulous sender to route large volumes of spam through your gateway.

You can define the way in which remote hosts attempt to connect to a listener. You group remote host 
definitions into sender groups. A sender group is a list of remote hosts defined for the purpose of 
handling email from those senders in the same way. 

A sender group is a list of senders identified by:

IP address (IPv4 or IPv6)

IP range

Specific host or domain name

SenderBase Reputation Service “organization” classification

SenderBase Reputation Score (SBRS) range (or lack of score)

DNS List query response

For more information on the list of acceptable addresses in sender groups, see 

. 

When an SMTP server attempts an SMTP connection with the appliance, the listener evaluates the sender 
groups in order and assigns the connection to a sender group when it matches any criterion in the sender 
group, such as SenderBase reputation score, domain, or IP address.

The system acquires and verifies the validity of the remote host’s IP address by performing a double 
DNS lookup. This consists of a reverse DNS (PTR) lookup on the IP address of the connecting host, 
followed by a forward DNS (A) lookup on the results of the PTR lookup. The system then checks that 
the results of the A lookup match the results of the PTR lookup. If the results do not match, or if an A 
record does not exist, the system only uses the IP address to match entries in the HAT.

Define sender groups on the Mail Policies > HAT Overview page.