Hi 

I am unable to create a new user for my domain, wkkoay[at]oncalled[dot]com. I tried to add new user, but it keep bring me back to first page and said that the user already exists. I am able to create a few other user email address successfully. It's not working just for this id. I also tried sending invitation link and import csv but is not working either. 

I found several similar problems here. But I don't think other users will use that email as secondary email. (need that user verification before able to set as secondary email) 

Can you please help me to check if any of my existing user used it for secondary email? If it's not the reason, what else might cause this?