Server Load Balancing: Alternate internal port than external
Just like NAT allows, let us choose an alternate port to redirect traffic to on the internal servers. Right now, whatever is external is internal, and it forces us to use NAT which kills any option to have high-availability and load balancing.
Kris Wilkinson commented
For example: mail.domain.com on port 2525 externally can redirect to port 25 on 3-4 internal servers. This way it's load balanced, and highly available. Right now we have to redirect to 2525 internally.