The biggest problems for mail delivery across the board are probably lack of correct helo/reverse ip lookup and being on an IP blacklist such as being on an ISP

The biggest problems for mail delivery across the board are probably lack of correct helo/reverse ip lookup and being on an IP blacklist such as being on an ISP dynamic address. I will bin those with my mail server as well. That is a very low bar but any naive mail server implementation on a home server is probably going to fail those instantly. DKIM and SPF are really good signals to have. I have DKIM, SPF, DANE(nobody cares), DMARC, MTA-STS, TLS (lets encrypt) which are all potentially useful in preserving reputation and repudiating emails spoofing my domain sent from other sources. I have never had issues getting throough to gmail. But for some mail systems, Microsoft in particular I think, they seem to penalise low volume mailers very heavily. You seem to need a certain volume for their systems to cache a reputation score and if you have a small vanity domain/mail server with very low mail volumes you may go straight to the spam folder.