Preserve original source IP with SNAT for SMTP

They use the source IP to tell whether this IP is allowed to send emails or do something else on the backend server. They have a huge list of these IPs, it's configured on the backend server and only these IPs are allowed to use the server.

I could control it on the firewall, allow traffic only from these sources to LB VIP, but the list is huge and they change it quite often, hence they want to manage the list on their own.