6th request - add a user to email not working

6th request - add a user to email not working

This is my 6th emergency ticket im sending! 

Its the same issue, no one is responding... 

I AM A NEW USER! 
We Have our entire company switched over to zoho mail
MY EMAIL IS DOWN FOR A CO FOUNDER!!! 

carter@day7m.com 
is not working, it is missing in the system. 
I cannot add a new one, it doesnt let me. 

I need to add carter@day7m.com to my users... 

Please someone respond... this is my 6th request and im getting in trouble over here with his email being gone!