With a couple of changes (highlighted in bold below) to you first rule, I think that should work assuming that your routing table directs a destination of 188.8.131.52 out the interface you wish:
iptables -t nat -A PREROUTING -p tcp -d 10.20.117.0/24 --destination-port 445 -j DNAT --to-destination 184.108.40.206:465
If you wish you can make your second rule more general where it will SNAT all LAN packets going out the public interface. In most cases this is what you would want:
PUBLIC_IF=<public interface, e.g. eth1>
iptables -t nat -A POSTROUTING -s $LAN_IP -o $PUBLIC_IF -j SNAT --to-source $PUBLIC_IP
Adjust $LAN_IP, $PUBLIC_IF and $PUBLIC_IP as needed.
If you are unfamiliar with CIDR notation, you can read about it here
. In a nutshell (quoting from the linked page):
In CIDR notation, the number of 1.s in the binary version of the mask are counted from the left, and that number is appended to the end of the base address following a slash (/). In the example here the subnet would be listed in CIDR notation as 192.168.1.0/24.