Hi! On this thread 7c/fakefilter#73 some random user asks to block ProtonMail and SimpleLogin email domains. This is really popular filter, so please, take measures! @mmso, @bartbutler, @Twikito
It's actually really easy to get past the custom domain issue.
If the domain is send-and-receive, it will need a SPF record to avoid getting blackholed by most mail providers. A TXT lookup for the SPF record would tie the custom domain back to the real provider.
Or even easier, you could look up the MX records to see what domain they point to.