Oh shoot. Good point. It's running on a Google VM instance, so I push outgoing SMTP through Mailgun. That's a factor I hadn't considered.

Incidentally, if you are getting flagged as spam, using a relay like Mailgun could be an option to explore. For postfix, the relevant option is 'relayhost', cf. postconf(5).

Also not the person you were replying to, but in most blocklists are ranges of what are known as dynamic IPs. i.e. those handed out to residential customers. Because most email spam comes from botnets running on peoples personal machines, these ranges are quite sensibly blocked.

If you want to improve your chances of not being blocked when sending email, get a static IP address for your home connection (if your ISP supports that) or host your mailserver on a server instance at a proper hoster.

I also run a personal mail server and haven't had any issues with delivery (that I know of). I'm using Mail-in-a-Box, which deserves all of the credit for keeping the box low friction to setup and maintain. I'm running it on a low-end VPS from a cloud provider I'm too embarrassed of to name, but it works (usually).

Edited: cleaned up some of the mistakes made when editing and moving text around, prior to posting.

Not the guy you were replying to, but I run my own email off of a DigitalOcean vps, and I have yet to have problems with spam filters. I didn't even have problems with spam filters before I set up DKIM, DMARC, SPF, etc.

I imagine if I set up a new IP on DigitalOcean today it might be a different story, but anecdotally I don't think I've actually observed instances where this has become a major issue.

I run eigenstate.org (and the associated emails) using physical hardware in a colo. OpenBSD's OpenSMTPD makes it relatively easy to set up, although simpler spam filtering would be appreciated.