Can't register iCloud email address


I would like to use an email address for my OGS account, but it doesn’t seem to work.

I enter the email address correctly, am told a confirmation email has been sent. But nothing arrives. Not in my inbox, not in spam, nowhere. Same thing if I click the button to resend the confirmation email.

If I switch to a gmail account, the confirmation email appears almost instantly.

Is this a known issue with the domain, or is there something I need to do?