one user with multiple domains--how do I set this up?

one user with multiple domains--how do I set this up?

According to the information I found it is possible to be just one user with multiple domains.I have a paid subscription, which allows me one user for multiple domains.

Let's say I have Cindy@red.com is my first email address.

I'd also like to receive the mail from my other domains, white.com, black.com, blue.com

They are all verified, If I go to users to add Cindy@white.com, the system tells me I can't add another user unless I upgrade my subscription.

Can someone please tell me how to go about this?  BTW, I've also done the change to the MX entries.