I was a FastMail user until Gmail’s launch and regret that I abandoned my free FastMail account and it was deleted. Not sure if I ignored email warnings and that was years before Twilio so there were no SMS reminders and iOS didn’t exist so there were no push notifications to try to give me a last chance! I wish I hadn’t lost years of personal email history, especially since there were probably photos there that I didn’t have anywhere else. Back in those days, data was expensive enough that they probably wouldn’t think of automatically archiving your account into a zip file in case you came asking for it later (when I asked, it was long gone). Now I wouldn’t hesitate to pay if I could resuscitate that account. Can’t really blame FastMail and now I use them for a business and love their service but it was a good lesson for me.

AFAIK you can use a custom domain with mailbox.org.

Exactly paying for your domain name is the safest way how to "own" email. I wouldn't expect people to use email without owning domain. What happens when your service of choice ends or changes rules?