If your IronPort is configured with multiple IP addresses on the same subnet and you wonder which one is used then Cisco has an answer for you:
Q: Which is the default used IP address (AUTO) if there are multiple IP addresses on the same subnet?
A: If there are multiple IP addresses configured within the same subnet as the default gateway, the IP address with the lowest number based on a c-string search will be used.
More details with the example are available at Cisco Email Security Appliance section of www.cisco.com web site.
So before adding a new one make sure that the default interface has been defined either by a content/message filter, deliveryconfig or alt-src host action. Assume that altsrchost CLI command should be good too.
P.S. Tagged this post as FreeBSD only because Cisco IronPort is based on FreeBSD