[Help] How to write iRule to select default gateway based source IP address?

Hi, all.   I have used BIG-IP LTM v12 and my information:   02 ISPs: 178.102.1.104 & 122.255.27.144 => 02 default gateway. 02 SMTP Servers: 192.168.10.101 & 192.168.10.102. BIG-IP is default...