Email already exists

Email already exists

Hello! I am trying to add a user/email to our custom domain with zoho (thenatureschool . org . au). I added  jodinka @ gmail.com  and she registered with Zoho as per the link. I then went to control panel and as the super administrator tried to create her email:  jodie @ thenatureschool.org.au  but it keeps saying that it already exists...but it doesn't?! I have already sent an email to support @ zohomail.com some days ago but have no reply.  Can you please help me. Thank you