Yeah, Mac's are shit. Nobody uses them as servers. After paying the inherited 10% Jobs tax, on top of normal x86_64 costs, they are still stupid expensive. You spend $500, the minimum for a Mac-mini, and get a 1.whatever GHz shitbox. I can spend $220, and build a headless 3.3GHz Pentium with 2.133 GHz FSB, and DDR4 RAM, that runs Fedora, CentOS, Windows, etc.
And yes, opening TCP port 25 to your home server can very quickly kill the system. Forget attacks, spam alone will cause millions, if not billions, of attempted email relays.