If the new users have an email domain that has not previously been added to the hive, then you will need to create a support case to have the new domain added to the internal domain list first.

That list controls what user email address domains can be added to the hive as a security mesure.

Once the domain has been added, users can either be created via the mail connector or added by an admin.

As with adding new users above the email domain needs to be added to the internal domains list first. If users are changing email domain and therefore email address, it is best practice to add the new email address to the current user so mail data can seamlessly flow into the correct user account. If the new email address is not added to the current user account then it's likely a duplicate user would be created that would need to be manually merged.