The email address is already registered with another account

The email address is already registered with another account

Hello! I have a free account and emails configured to work with our domain. I added couple of email users which works fine. Problem is that I can't add one particular email. When I try to add it I'm getting following error: "The email address is already registered with another account which is not a part of this organization.". Only thing that comes in my mind is that someone of the previous web admins did that. What can we do about this?