Cisco Cisco Email Security Appliance C160 Mode D'Emploi

The appliance attempts to verify the sending domain of the connecting host via DNS for incoming mail. 
This verification is performed prior to the SMTP conversation. The system acquires and verifies the 
validity of the remote host’s IP address (that is, the domain) by performing a double DNS lookup. A 
double DNS lookup is defined as 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 appliance then checks that 
the results of the A lookup match the results of the PTR lookup. If the PTR or A lookups fail, or the 
results do not match, the system uses only the IP address to match entries in the HAT and the sender is 
considered as not verified.

Unverified senders are classified into the following categories:

Connecting host PTR record does not exist in the DNS.

Connecting host PTR record lookup fails due to temporary DNS failure.

Connecting host reverse DNS lookup (PTR) does not match the forward DNS lookup (A).

Using the sender group “Connecting Host DNS Verification” settings, you can specify a behavior for 
unverified senders (see 

You can enable host DNS verification in the sender group settings for any sender group; however, keep 
in mind that adding host DNS verification settings to a sender group means including unverified senders 
in that group. That means that spam and other unwanted mail will be included. Therefore, you should 
only enable these settings on sender groups that are used to reject or throttle senders. Enabling host DNS 
verification on the WHITELIST sender group, for example, would mean that mail from unverified 
senders would receive the same treatment as mail from your trusted senders in your WHITELIST 
(including bypassing anti-spam/anti-virus checking, rate limiting, etc., depending on how the mail flow 
policy is configured).

With envelope sender verification, the domain portion of the envelope sender is DNS verified. (Does the 
envelope sender domain resolve? Is there an A or MX record in DNS for the envelope sender domain?) 
A domain does not resolve if an attempt to look it up in the DNS encounters a temporary error condition 
such as a timeout or DNS server failure. On the other hand, a domain does not exist if an attempt to look 
it up returns a definitive “domain does not exist” status. This verification takes place during the SMTP 
conversation whereas host DNS verification occurs before the conversation begins — it applies to the IP 
address of connecting SMTP server.

In more detail: AsyncOS performs an MX record query for the domain of the sender address. AsyncOS 
then performs an A record lookup based on the result of the MX record lookup. If the DNS server returns 
“NXDOMAIN” (there is no record for this domain), AsyncOS treats that domain as non-existent. This 
falls into the category of “Envelope Senders whose domain does not exist.” NXDOMAIN can mean that 
the root name servers are not providing any authoritative name servers for this domain.

However, if the DNS server returns “SERVFAIL,” it is categorized as “Envelope Senders whose domain 
does not resolve.” SERVFAIL means that the domain does exist but DNS is having transient problems 
looking up the record.

A common technique for spammers or other illegitimate senders of mail is to forge the MAIL FROM 
information (in the envelope sender) so that mail from unverified senders that is accepted will be 
processed. This can lead to problems as bounce messages sent to the MAIL FROM address are 
undeliverable. Using envelope sender verification, you can configure your appliance to reject mail with 
malformed (but not blank) MAIL FROMs.

For each mail flow policy, you can: