Carrier Central Sign Up Not Working

I have been using Carrier Central for over 2 years with no problems. However 2 weeks ago I went to book in a delivery of a pallet and although I was ‘signed in’ it was asking me to ‘sign up’ as though it was my first booking. I have signed up 4 times and each time received an email saying it will not accept my email address because it is from a ‘public domain’…but I am using the email I use for my Amazon FBA Seller Account. I have also opened cases and sent emails to the correct department but just receive an automated response with troubleshooting advice, no actual response to my email. I can’t see why the sign up is not working and I can’t see why I need to sign up anyway as I have already been through this process over 2 years ago. Would be very grateful for any advice as I have been trying to get a response for nearly 3 weeks now.

Many thanks for reading