This week I configured my home server as mail server.
Since port 25 is blocked inbound and outbound, I played with the settings to change the listening port to 2525. It has a dynamic IP, therefore I used a DDNS service to set the A record of the domain's DNS zone.
Then I configured another nethserver I have access to and that has a full featured fixed IP internet connection as relay to my domain:2525. Actually I even modified the php files to make it possible to set the SMTP port directly on the UI.
My domain name's DNS zone obviously have MX records that points to the fixed IP server.
Mails are sent via a smarthost, my ISP's SMTP server.
Awkward and a bit ghetto, but works perfectly and I learned a lot.