My bad, I wasn’t clear enough. I actually meant Proton Bridge has no mobile version, meaning you can only use the official app you mentioned, but not any third party apps like K-9 Mail/Thunderbird or FairEmail.

I’ve been on Posteo for years now, they’ve been rock solid. As for Tuta.io, the issue specifically is no IMAP support, which is what third party email clients need to function.

I thought I’d also mention Protonmail, which might look good, but is in a similar boat. They technically do provide IMAP support, but not in a way that matters. If you wanted to connect Protonmail to Thunderbird or, if you’re an insane person, Microsoft Outlook, you’d need another app running on your PC along with your email app called Proton Bridge, which just sounds like a hassle. No mobile version either.

I’d say stick with Posteo. Alternatively, if you want to use your own domain name, I’ve heard good things about mailbox.org.