I need to change my account email address but Box complains it is unvalidated

I am going to be closing down the domain I used when I first created my Box account, and need to edit the email address associated with that domain which I used when setting up my Box admin account.  I understand that I need to add a new email address and then make it the default, but Box won't let me add the new address and tells me it is unvalidated.  How do I validate it?