I've hosted my own mail server for a while. I've gone the Postfix/Dovecot route, I've used qmail, etc. My current setup is WebMin/VirtualMin. It works really well, and will run on the cheapest DO droplet.
During if you want to to from 1-2 spam a day to 1-2 a month you might want to block the garbage domains like click, link, party, top, webcam, xyz, stream etc. Probably worth enabling a DNS based block list.
So an apt get or two, 2-3 lines in a config file. Seems trivial to me, most every mail server HOWTO mentions them, should be just a cut/paste.
Sure 9-12 months from now it won't work as well, thus updating SA periodically, just like anything else internet facing.
Fighting spam effectively is not trivial.