Cisco Cisco Email Security Appliance C160 Mode D'Emploi

Extending the basic syntax, HATs in AsyncOS support the ability to create named sets of remote host 
definitions; these are called sender groups. Named sets of access rules combined with parameter sets are 
called mail flow policies. This extended syntax is illustrated in 

The order that rules appear in the HAT is important. The HAT is read from top to bottom for each host 
that attempts to connect to the listener. If a rule matches a connecting host, the action is taken for that 
connection immediately. 

Predefined and custom entries you place in the HAT are entered above the final “ALL” host entry. 

For all public listeners you create, by default, the HAT is set to accept email from all hosts. For all private 
listeners you create, by default, the HAT is set up to relay email from the host(s) you specify, and reject 
all other hosts. 

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.

Mail Flow Policies of the HAT allow you to control or limit the rates at which the listener will receive 
mail from remote hosts. You can also modify the SMTP codes and responses communicated during the 
SMTP conversation. 

The HAT has four basic access rules for acting on connections from remote hosts:

ACCEPT

Connection is accepted, and email acceptance is then further restricted by listener settings, including 
the Recipient Access Table (for public listeners).

REJECT

Connection is initially accepted, but the client attempting to connect gets a 4XX or 5XX greeting. 
No email is accepted.