Another alternative to Posteo is Mailbox.org [1]. Personally, I use Soverin [2], but that's mostly cause I get it for free with Freedom Internet plus its hosted in The Netherlands. And I can always leave.
Posteo also runs on FOSS and is being endorsed by the FSF - if that matters. They also do 2FA, via TOTP (which many apps support). They offer a migration tool that downloads email from your other accounts.
Both of these email provider options support TOTP so I omitted it; and TOTP ≠ U2F/WebAuthn with a USB token. I have an OnlyKey, but it’s mostly full of TOTP keys already because applications are not supporting WebAuthn. Posteo does not support this, and last I emailed them, they said they do not release information about future features.
Never heard about Posteo. Too bad it does not support custom domains.
Posteo pros: comes with calendar and contacts via WebDAV, not registered in the US (Germany is part of 14 Eyes, but not 5 or 9, and the EU is better about privacy), cash payment option
Purelymail pros: more storage @ $10, custom domain support, security key 2FA (though unclear according to docs if this is WebAuthn/FIDO2 or Yubikey vendor lock-in)
Neither: cryptocurrency payment option