Hi I tried to change my mail to a proton.me mail as I’m moving away from google but got hit by an error stating the domain was not allowed. Contacted Proton and they had an FAQ with this in: 

“Email template to send to websites that block Proton Mail

To: customer support email address
Subject: Unable to register with my Proton Mail email address

Hello,

I would appreciate your help in registering my account with your service. I was unable to use my proton.me email address during the sign-up process.

The block of Proton Mail on your domain must be a mistake. Tens of millions of people globally use Proton Mail, which is a secure email provider based in Switzerland. The company was created by former CERN and MIT scientists to provide easy-to-use encrypted email. I believe it’s important not to discriminate against users who prefer a privacy-focused email service.

Thank you for your assistance.

Best regards,

Me”

Can you please elaborate on why the domain is blocked or maybe unblock it?

Thank you!

Probably has something to do with the influx of spam / advertising bots that may've used the service.