Alias Email Id already registered by user not belongs to your organization.

Alias Email Id already registered by user not belongs to your organization.

I'm trying to add email alias "milej.starsocks@scamper.pl" into my account but it claims that "
Alias Email Id already registered by user not belongs to your organization.
"
How con I fix that? Probably I registered user with that email as separate zoho account but I'm not able to login because I have no access to that email address ...