Transferring Domains from one Tenant to Another

Hi

I need to move my domains from one tenant to another for Mail Protection (US EAST to EU)

I successulyy added our primary .com domain to our new tenant and mailflow is setup, and working as expected, even though it remained on the source tenant for a while after. The .com domain has now been removed from the source tenant.

However, our .co.uk domain will not add to the destination tenant. It says it already exists with another customer. Even though the .com domain added fine?

I tried deleting the .co.uk domain from the source tenant and adding it again, but the destination tenant still stated the .co.uk was in use by another cuastomer.

What do I need to do to allow the destination tenant to add the .co.uk domain?