If IMAP is enabled on a provider, that provider can access your emails, unless you’ve encrypted the content of the email itself (with something like pgp or gpg). Proton only has access to emails in transit and after that, it can no longer access your email as it’s entirely encrypted. Since Proton doesn’t save the emails in transit, it has zero ability to provide those emails even if given an enforceable subpoena. Other providers that use IMAP can and do have access to your emails and can give them to a government authority if given an enforceable request.

The difference is the data at rest protocols on different providers. Proton has zero access encryption for data at rest. It only has access for data in transit and its ephemeral in that once it’s done with that transaction, it no longer has that data.