Exact details depend on whether you are using NAT or not. Access-list on Private side should look something like this:
Assume your mail server is 192.168.22.22
ip access-list extended inside-to-public
permit ip host 192.168.22.22 any
deny tcp any any eq 25
! Any other outbound restrictions here
permit ip any any
Then apply this ACL to your outbound policy-class on the LAN side facing the public, something like...
ip policy-class Private
nat source list inside-to-public interface [interface name here] overload
Of course you'll need the appropriate reciprocal port-forwarding to allow inbound mail from the public to your mail server as well. Post your existing config if this doesn't help.
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