I haven’t used Hushmail, so I can’t comment on your situation. For Proton Mail, I pay for a custom email domain but I use SimpleLogin to proxy emails through an email alias so that my real email address doesn’t leak as easily, and I can identify the source of the leak if it does happen (and easily cut it off). Most web services have no issue with the simplelogin.com alias, but there’s a small number that do denylist the domain, so I need to use some other email address instead (either a throwaway or a dedicated address from my real domain). If you use the free Proton email domain, I doubt anyone would block it — it’s too widely used and trusted.