LTM-New net segments LB+SNAT

Specifying two pools in the iRule won't work. I'd recommend you change your pool_gateway_internet_2 pool to use all six gateways. Then only specify that pool on your iRule for a match against your email server IP address:

when CLIENT_ACCEPTED {
if {[IP::addr [IP::client_addr] equals Private_IP/32]}{
 pool pool_gateway_internet_2
 snatpool snat_mail_server_to_6_providers
 } 
}