Symptom:
In AsyncOS version 8.0.1 and earlier 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.

With 8.5.5 version, default interface will be selected based on the numeric IP address. The first IP Address (normal numeric counting) in the list that resides on the subnet in question will be used.
 
The order of IP interfaces used will no longer be done with a c-string search, it will be done based on the numeric order of the IP addresses.

Conditions:
Email Security Appliance with AsyncOS 8.5.5.