Currently thinking about either Tuta or Mailbox.org, but to be honest, I don’t even know many providers.
Something I do not like is limit on number of addresses with a custom domain. Mailbox is at least a bit reasonable with 50 on Standard, but Proton’s 10 on Mail Plus is disqualifying for me.
Tuta has unlimited number, but doesn’t support PGP.
So I plan to check out whatever you guys use.


You spend an hour or two setting up DKIM , SPF, and DMARC, so that Gmail/Hotmail/etc can validate that you are who you say you are.
You still can end up in spam. Gmail’s spam filter is a big black box. I doubt even Google fully understands it.
DKIM/SPF/DMARC add extra legitimacy to your message, but you can certainly still end up marked as spam if Google thinks you’re sending it.
One line emails, emails that just have links, emails with generic subjects, emails with just a subject and no text body, emails with no subjects - all that massively shoves Google’s spam detection towards spam. But that generally holds true with all providers as all the big players are using some form of Bayesian filtering (and so am I, with SpamAssassin on my incoming). If you send spammy-looking stuff, you’re gonna get marked as spam.
There’s also IP address reputation, IP warm-up (not necessary for personal servers, though), missing things like list-unsubscribe (again, not necessary for personal servers), and of course a touch of “this message looks like other messages that users have marked as spam” on an email from your own server with a message to yourself. There’s so much more than just a Bayesian spam filter in Gmail’s black box.
Also if your ISP puts your IP on the PBL. They also don’t like it if your reverse lookup doesn’t match your domain. I was able to send from my IP for a while and then Google stopped delivering my mail with the PBL given as the reason. Other destinations may be less picky but Google has a lot of requirements some of which are out of your hand unless you have a static IP.
I think all email services should be required to list their delivery requirements publicly.