An alternative I have setup myself is:
- Protonmail with PGP keys (public and private)
- Addy.io with my public PGP key
I did this just as a test. When my protonmail subscription ends and I change email provider to one with IMAP/SMTP support I will set the same setup (with new keys). That way, as soon as Addy receives any incoming email, it gets encrypted and forwarded to my email provider which will store the email encrypted.
So I will reply (or not) those emails with a PGP trusted client configured with my PGP keys (public and private) and technically the mail provider should not be able to read those emails (unless I missunderstood anything, because I did not test and I will not have that setup until my proton subscription expires).
This is much better explained from @djkilla in this thread (which recommend to fully read if someone is interested): Is Addy's Lite Plan worth paying for? - #6 by djkilla .
This way I think the trustability of the IMAP/SMTP service does not matter (but feel free to correct me if I am wrong).