I was not specifically talking about tutanota at all. It was more of a generalisation. You are right, nobody does know with tutanota or proton mail or others in regards to free user vs paid users.
The reality is its more about whether you want to be able to use standard email client or not. If you use Tuta you’ll have to use their email client/webmail and accept its user experience limitations so I suggest trialing that first. You may find things you’ve been used to doing in Gmail, Outlook or Thunderbird that aren’t possible. Eg for a long time you could not make subfolders.
Mailbox.org while less of it is encrypted, at rest eg email headers, calendars, contacts its important to realize email itself isn’t very secure. If your recipient is not on Tuta, there’s no E2EE once it leaves Tuta’s servers. Eg if you were to email a gmail account with either Tuta or Mailbox.org an unencrypted copy can sit on Google’s servers regardless.
Mailbox does have the advantage of allowing say a Proton Mail user to encrypt an email to you however with your PGP key.
I do not think password protected emails work in practice either, as most businesses will simply not open those. If it’s someone you personally know, then better off communicating on Matrix or Signal if you require a completely closed communication mechanism.
On a side note, amongst normies I’ve never seen a Tuta user, where as I have seen Proton users, so I would think they’re “larger”.