They do have e2e for emails. Any emails between Proton Mail users are always e2e encrypted, as are any emails others send you which they’ve encrypted with their own maio client. If someone sends you an email unecrypted (most email is), then Proton will encrypt it for you and put it in your inbox. They can’t read it after that, but there is some trust required that they don’t store/look at the unecrypted email before then.