Free and Open Source Software

Any provider that provides access via IMAP/SMTP works fine with a lot of email clients. I personally use mailbox.org.

As clients you can use Thunderbird, Geary or Evolution on the desktop and FairEmail or K9 (which will become Thunderbird for Android) on Android.

I recommend getting your own domain name and using provider like Migadu.

Take a look at https://www.privacyguides.org/en too.

Tuta doesn't allow third part client. In Linux I personally use Betterbird that is a fork of Thunderbird and on Android I use K-9.

How about kolabnow.com

Their platform itself is open sourced (see https://kb.kolabnow.com/faq/what-does-kolab-now-run and https://kolab.org/), they adhere to standards, and they are recommended for privacy reasons as their servers are in Switzerland.

You can create as many email addresses as you want and they all go to your standard inbox, which is great not only for being able to just cut-off spammy websites, but also for privacy, as you won't be connected to a single address.

I'm using posteo with Geary on linux and K-9 on Android.

So! Personally, I would recommend against anything that requires you to use their platformed client like Tutanota and Proton. Don't get me wrong, they're both excellent services, but ultimately, they sit antithetical to the point. Making email secure and private and good is hard, bordering on impossible. To accomplish that, you have to jettison core components of email protocols out that stand in the way of making it a secure and private and good messaging service, core components other email vendors will be making heavy use of. This can be fine, as I'm sure you've experienced with your tuta account, most of the time, up to the point where you have to do something weird like "email your parents who don't really understand technology"

Instead, I'd recommend going the complete opposite direction. Find you a vendor like https://mailbox.org or https://posteo.de whose offered services are "We take your money, give you an inbox, and we don't sell your data because that's what we take your money for." After this, there are things you can do to make your email almost secure, almost private, and almost good, but nothing will ever compare to sending messages over a good protocol like Signal Protocol, XMPP, or Matrix (all of which use shared protocols under the hood to ensure secured messaging with forward secrecy).

The bottom line, effectively, for me, is that I want to always be looking for platforms that are making as best use of standards-compliant protocols as possible rather than presenting any form of platform lock-in. It makes it easier to recover from a disaster this way because there will be other platforms with the same protocols available, and interoperability between disparate platforms will be much easier.

I would need someone to confirm that this is a good idea, but have you considered joining a Pubnix? Many of them come with email, as well as the standard web hosting and SSH.

Personally, I also rather liked Disroot before I switched to ProtonMail (which doesn't support IMAP on the free tier).

It really depends on your needs and what's important for you in the services you use.
For personal use I go with disroot and k9 for client. Here is a quite detailed overview
But they also more than email service: cloud service, collaborative tools, messenger service, calendar and tasks,, paste bin...

Thanks for asking. I am looking for a new provider myself. I am interested in people's suggestions.

Personally considering using my own domain this time. Take a look at Proton Mail, and Fastmail. Namecheap has mail too and you can buy domains there as well. None of these are free but that is the price of being the customer not the product.

It is also possible to get a business account with Google which presumably they would not data mine. I would verify though.