I am not sure what tech level you are asking at so if I am too basic/not basic enough, please excuse me.
Fastmail is a great email service provider. You pay for the service so the idea is that your personal data/privacy is not sold on as Fastmail is financed by customer subscriptions. I hold the opinion that this is likely true.
While I was a customer the service was great. Great spam protection and speed. Easy to cancel subscription in your account page etc. The reason I left is that Fastmail do not do TNO(Trust no one) end to end encryption. Their service appears secure in all the usual ways like https etc. But they have access to your emails if they so wished.
The provider I am with now Protonmail provides TNO encryption. So the idea is I am the only one that can view my emails as I hold the encryption key not my email service provider if I send from and recieve from a Protonmail account. The downside is that if I lose my key, then my email is gone for good. Now if I send email to a non Protonmail account then then I believe the email is only sent via TLS in transit and not encrypted so the other email services can read the email, unless I use the password option or PGP.
I looked at the Tutanota email service too but they did not support IMAP clients like Thunderbird to collect email. You could use their apps.
That is my take on it. I am not an expert so do not rely on my thoughts/understanding. There is a lot of discussion on how secure any of these services are really and the way they implement things. Just like the Crypto AG story, you never really know. It all depends on your personal preferences and threat model/privacy stance.