You can do this, but not as a standard, public access MTA, only a private-use MTA. e.g.: via dyndns.com's mailhop relay service.
It's probably easiest via pf(4). Here's an example excerpt from a redirected port 2525 to port 25 on the loopback interface:
Code:
rdr pass log on $external_nic proto tcp from any to any port 2525 \
-> 127.0.0.1 port 25