EMAIL ID ALREADY EXISTS (Error)

Hi, I can't seem to be able to add a new user email account named "admin" for my custom domain. It says the ID already exists and is part of another organization. I tried several times but in vain.

I was able to create other email IDs such as " support@customdomain.com" and " test@customdomain.com"... Please clear the " admin@customdomain.com" so that I may create it. It does not exist anywhere else!

Thanks