Many exchange experts prefer multiple Edge servers with external IP then creating multiple MX records. With same or different priority.
But what if you don’t have enough real IPs and you want High availability for Edge Server Role?
I have tried the following approach and its working nice:
I have used windows Network load balancing between edge servers then make the external firewall nating the inbound traffic to external IP to the virtual IP of the NLB, and make outgoing traffic from each edge server to go through the external real IP