I used the process described in this blog post to be able to deliver to Hotmail: https://blog.stickleback.dk/getting-off-hotmails-blocklist/ (not my blog). I got the same strange automated responses as this guy got, but I got off their blacklist after a couple of hours.

More generally:

- I made sure my server's IP addresses weren't on any blacklists.

- I host my mail server with a reputable hosting provider (RamNode). Stay clear of companies like OVH that have reputations for poor abuse handling.

- I have configured SPF, DKIM, and DMARC.

- I registered my server with https://www.dnswl.org/, a whitelist for legitimate mail servers.

- I don't send spam or other bulk e-mail, only personal correspondence.

I also tried sending e-mails to a couple of people I know who use Gmail. I asked them to mark my mails as 'not spam' if they went into their junk folders. I don't know how much this helps "globally" in the Gmail system, but at least it'll teach Gmail not to mark my mails as spam for those people.

I had used my current personal domain for e-mail for a couple of years but I paid a company to host my e-mail for me. It might have helped that my domain had already built up a good reputation in big mail companies' spam filters, but who knows... It definitely helps, however, that my domain wasn't brand new. Newly registered domains are often penalized by spam filters. A poor choice of TLD can also hurt delivery: I know that many mail admins penalize or out-right block many of the new gTLDs like .top and .download.

I am not discounting those who complain about deliverability, but so far I haven't had much trouble with my low-volume, personal mail server.