I think this is a training issue. You need to make the list of companies then retrieve the contacts and remove the company records from the list. This will allow you to export the email addresses for all the contacts at companies marked as customers. You can not currently search a field on a client record while retrieving the contacts in one go.
If you wish to purchase a remote training session to help you learn how to do this I would suggest you purchase an incident. Give us a call on +44 0345 2608801