Can't use my personal email domain

I’m trying to use my personal email for registering, but I get an error from Discorse:

Email is not allowed from that email provider. Please use another email address.

I’m trying to use a personal domain `****@conto.dev` that I have hooked into Fastmail. Does anyone know what might be blocking registration? I’ve never seen this before on other sites, including other Discorse forums.