It is common that companies have non-executive directors, and that they work without corporate email accounts. They could use gmail or another personal email accounts so they might want to blacklist free email domains, but have special dispensation for these exec's email addresses. So blocking all free email domains, with an additional granularity to allow certain addresses on a whitelist.
Admindaphnezhao (Admin, Box) commented
Exception handling for external collaboration planned for H2 will address this.