FWIW I've been running my own SMTP client/server for many years (I wrote them myself for my purposes, it's hobby stuff), with the proper SPF records it gets acc

FWIW I've been running my own SMTP client/server for many years (I wrote them myself for my purposes, it's hobby stuff), with the proper SPF records it gets accepted by Google & co. no problem. Only residential Internet IP range are generally blacklisted by default by antispam filters.