trouble add user, error is "The username which you have entered belongs to a different deployment/region"?

trouble add user, error is "The username which you have entered belongs to a different deployment/region"?

I'm adding users to our domain salvagentemilano.it MX hosted by you at zoho.eu.

When I try to add a new user sometimes I get the "The username which you have entered belongs to a different deployment/region" error message.

I've added a screenshot to explain better.

I'm a little bit in hurry, these users is coming from another hoster and now people is trying to write them.

Nobody know a solution?