"address is already registered with another account which is not a part of this organization" for my own domain.

"address is already registered with another account which is not a part of this organization" for my own domain.

Hi,

I am trying to add the email of a collegue to our corporate mail (@stella.mobi) as an alias to her current address, but I keep getting this message:

The email address is already registered with another account which is not a part of this organization.
 
The email I am trying to add is either "deborah" or "debora.couto".

The first thing I did was to invite her, but since she could not log in I tried to delete the invitation and add her manually.
Since this did not work, I added her as "deb" (so she could have something to use) and now I am trying to add one of these emails as an alias.

It's been a couple of months since this happened, and I still can't add her.

Is there anything I must do to release one of these two addresses?

Best regards,
Diego