Not able to create an email id - Email already exist.

Not able to create an email id - Email already exist.

Hi Team 

I have activated a free version of zoho mail and iam trying to create an email id "  dgm@mindmentoracademy.com " where it shows an error that its already created but, iam not able to see the same in our email user list. There is no any email alias for with this name.

Can you please guide us on how to fix this issue.   
Thanks
Balaji.S