In reply to Daniel Spreadbury.

I've had the same problem when attempting to set up accounts for family members. All have had gmail.com emails, and after pressing submit an error comes back ref the email address. I've even attempted disabling my adblockers to no avail. I suspect that the error may happen because I use the Duck Duck Go privacy extension. The Steinberg website contains a horrible amount of marketing trackers but users should still be able to use the site whilst only allowing essential functional cookies.