Proton and Tutamail don’t support IMAP as they use their own system for encryption and whatnot, which means emails are more secure (as long as everyone uses Proton and Tutamail, which the majority of people don’t). I also find their built-in clients to be quite slow. Disroot, Autistici, and mailfence all support IMAP. There’s a bunch more good ones (both free and paid) that support IMAP. Lots of people uses mailbox.org, which is based in Germany, and some people use Fastmail, which is based in Australia.