Cisco Cisco Email Security Appliance C160 Mode D'Emploi

 shows the suggested settings for implementing sender verification:

In the GUI, click HAT Overview on the Mail Policies tab. A list of existing sender 
groups is displayed. To enable and configure host DNS verification for the 
SUSPECTLIST sender group:

On the HAT Overview page, click SUSPECTLIST in the list of sender groups. 

UNVERIFIED

SUSPECTLIST

THROTTLEMORE

THROTTLED

Prior to SMTP conversation:

Connecting host PTR record does not exist in the DNS.

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

ACCEPTED

Envelope Sender Verification during SMTP 
conversation:

- Malformed MAIL FROM: 

- Envelope sender does not exist in DNS.

- Envelope sender DNS does not resolve.