Jennifer is correct as far as statically natting your 3 email servers to the single external interface - it's not possible.
However - let me get this straight.
* You have a single load balancer server that you wish to RECEIVE email on, which will then deliver that email to 3 internal servers?
Just port map port 25 on 63.xx.xx.153 to port 25 on 10.21.31.16 (your load balancer) Something like this:
static (inside,outside) tcp 63.xx.xx.153 smtp 10.21.31.61 smtp netmask 255.255.255.255
* You have 3 internal servers that you want to SEND email from directly to external servers and have them reverse DNS to your external IP 63.xx.xx.153?
Allow your 3 servers to connect to anything on port 25 through your global nat policy, something like this:
access-list acl_out extended permit tcp EMAIL_SERVER_GROUP any eq smtp
I assume you already have an internal to external internet connection (global NAT policy), so this will mean external receiving email servers will see your internal servers connecting to them from 63.xx.xx.153.
What wouldnt be possible is having your 3 mail servers sending AND receiving on your single external IP address, but the above scenario is fine because you are only receiving email to one server (your load balancer). The 3 sending servers only need to be allowed to send out the firewall.
Your better off posting your current config and then Jennifer or one of the other experts will be able to tell you the exact commands you'd need to achieve this..
NOTE: You should really also consider having your load balancer sitting in a DMZ, as that is essentially open to the world, so in a production environment that should be sitting in a completely separate network, if you have extra physical ports on your firewall then plug your load balancer into that
