How to Configure MX records for a subdomain on ProtonMail
If you are configuring a subdomain to use ProtonMail, it works well though the ProtonMail instructions aren't clear. Here's what you need to do.
Say your subdomain is secure.domain.org
The ProtonMail instructions indicate that you should use @ for the host, this is equivalent to domain.org, so it's not what you want. Depending on your how your name server manages your DNS records, instead of @ you want to use either secure or secure.domain.org
I've also noticed that it takes longer than I expected for the change to propagate, so make the change to your DNS MX record, wait an hour or so and check your ProtonMail dashboard (Settings > Domains > Custom Domains).
Propagation could take longer depending on the TTL setting for this MX record.