Inside a tenant, server-level object types are forbidden and x:Tenant reads
return only the caller's own tenant, which it can't change. Registry writes
refuse links across tenant boundaries in both directions, give a new
principal its domain's tenant, move a domain's principals and DKIM keys with
it into a tenant, refuse moves out of a tenant while its people remain, and
refuse creates past a tenant's count limits with overQuota and
limit.tenant-quota.