I see a combination of issues with Posteo, mailbox.org, fastmail, and many other commonly suggested providers that made me steer away from them: 1) they don't have a free tier, and 2) they recycle your email address if you stop paying. Using a custom domain for email is not generally considered a best practice for privacy, so I avoid that too. This greatly reduces the number of options available to me.
Edit: my issue is not with 1 alone. The problem is the combination of 1 and 2. It means that if you're unable to pay for the service for any reason in the future, your old email address can be taken over by someone else.