when trying to change email on account, I get a message that the "new" email is already in use

I am trying to change the main email that my account is registered under, as it is under a previous employee's email address. I want to change it to my email. 

Whenever I share documents, they appear as if they are coming from the prior employee's email.  I have changed my profile name, so that it has my actual name, but I haven't been able to change the account email address successfully. 

I have removed my email as the secondary email on the account.  I have also deleted an unused Adobe account registered under my email.  But when I try to change the active account's email to my address, it tells me 'email already used'.  

I tried logging in to Adobe using my email, and got a notice that my account had been deleted, which is as it should be.  So, why can't I set my email address as the primary one on the active account?  

Thanks in advance for any help you can offer!