SPF and DKIM are pretty much required now, as are TLS/SSL. From there, it turns into a dice roll. Gmail is terrible about this; they have a totally opaque and very frustrating engine that sometimes filters messages into a junk folder and sometimes doesn't. Outlook.com uses a somewhat more traditional internal RBL, but they are happy to block network segments and they don't offer any way to query their blacklist or request removal from it, so you could do everything right but a neighboring VPS or IP will get you blacklisted anyway. Comcast will simply accept delivery of messages and then disappear them depending on an arrangement of the stars that I haven't quite figured out yet. And those old sbcglobal/Yahoo services users... just, uggghh.

A popular solution is to rely on a third-party service to handle your outbounds. Sendgrid specifically is really bad, they carry way too much junk traffic, so don't go with them if you decide to try this out.

I have a post banging around in my head that's titled "Email is fractally broken", and getting outbound non-spam messages to reliably land in other people's inboxes would be a significant part of that writeup.