Cisco Cisco Email Security Appliance C170 Guia Do Utilizador

Use the Import Table button on the Destination Controls page or the 

destconfig -> import

 command 

to import a configuration file.You can also export your Destination Control entries to an INI file using 
the Export Table button on the Destination Controls page or the 

destconfig -> export

 command. 

AsyncOS includes the 

[Default]

 domain control entry in the exported INI file.

You can use the 

destconfig

 command in the CLI to configure Destination Control entries. This 

command is discussed in the Cisco IronPort AsyncOS CLI Reference Guide.

A “bounce” message is a new message that is sent by a receiving MTA, using the Envelope Sender of 
the original email as the new Envelope Recipient. This bounce is sent back to the Envelope Recipient 
(usually) with a blank Envelope Sender (MAIL FROM: < >) when the original message is undeliverable 
(typically due to a non-existent recipient address).

Increasingly, spammers are attacking email infrastructure via misdirected bounce attacks. These attacks 
consist of a flood of bounce messages, sent by unknowing, legitimate mail servers. Basically, the process 
spammers use is to send email via open relays and “zombie” networks to multiple, potentially invalid 
addresses (Envelope Recipients) at various domains. In these messages, the Envelope Sender is forged 
so that the spam appears to be coming from a legitimate domain (this is known as a “Joe job”). 

In turn, for each incoming email with an invalid Envelope Recipient, the receiving mail servers generate 
a new email — a bounce message — and send it along to the Envelope Sender at the innocent domain 
(the one whose Envelope Sender address was forged). As a result, this target domain receives a flood of 
“misdirected” bounces — potentially millions of messages. This type of distributed denial of service 
attack can bring down email infrastructure and render it impossible for the target to send or receive 
legitimate email.

To combat these misdirected bounce attacks, AsyncOS includes Cisco IronPort Bounce Verification. 
When enabled, Cisco IronPort Bounce Verification tags the Envelope Sender address for messages sent 
via your Cisco IronPort appliance. The Envelope Recipient for any bounce message received by the 
Cisco IronPort appliance is then checked for the presence of this tag. Legitimate bounces (which should 
contain this tag) are untagged and delivered. Bounce messages that do not contain the tag can be handled 
separately.

Note that you can use Cisco IronPort Bounce Verification to manage incoming bounce messages based 
on your outgoing mail. To control how your Cisco IronPort appliance generates outgoing bounces (based 
on incoming mail), see 

When sending email with bounce verification enabled, your Cisco IronPort appliance will rewrite the 
Envelope Sender address in the message. For example, MAIL FROM: 

joe@example.com

 becomes MAIL 

FROM: 

prvs=joe=123ABCDEFG@example.com

. The 

123...

 string in the example is the “bounce 

verification tag” that gets added to the Envelope Sender as it is sent by your Cisco IronPort appliance. 
The tag is generated using a key defined in the Bounce Verification settings (see 

 for more information about specifying a key). If this 

message bounces, the Envelope Recipient address in the bounce will typically include this bounce 
verification tag.