If you have Proton Premium point your domain to SimpleLogin and use it. Its included with Proton Premium. Its helped me root out 2 places so far that have sold my email address or were compromised and failed to disclosure.

I moved off to zoho

Much cheaper than proton and offers much more.

They’re not doing like proton and close basic stuff like IMAP and SMTP as a way to force you on the official apps

I especially love the feature where you can bounce emails based on domains, keywords or TLDs. My spam folder is finally empty. IMHO bounce back spam is much better, as the spammers get a response that the address is invalid and hopefully stop wasting their limited computing resources on that address.

Zoho is not open source, but proton is a “fake” open source that is mostly used for marketing: they opened only the UI, which communicates with a proprietary protocol to a proprietary server - useless. They also reject or ignore any pull request on GitHub.