This Knowledgebase article describes a known issue related to multiple IP addresses used in an Exchange environment and their effect on the e-mail delivery process if 'Reverse DNS lookup' is used to secure the environment.
If you are receiving e-mail messages directly from other domains on the Internet, you can configure your SMTP virtual server to perform a reverse Domain Name System (DNS) lookup on incoming e-mail messages. This configuration ensures that the sending e-mail server's IP address (and its fully qualified domain name) matches the e-mail message sender's domain name.
Although 'Reverse DNS lookup' is a configurable option, SMTP servers frequently use it for authenticating the origin of e-mail. When using management IP addressing on a pair of servers that run MS Exchange and are protected by Neverfail, the first IP address in the IP address list will be the management IP address. Since SMTP inserts the first IP address of the sending server in the header of an SMTP message, reverse DNS lookup will fail to validate the address and possibly mark the e-mail as spam.
Neverfail recommends that management IP addresses not be used in an Exchange environment because use of management IP addresses may prevent authentication by your gateway or firewall unless these devices are appropriately configured.