[otrs] How to send e-mails to all customers in the same time

Hello all, I started on OTRS 1 week ago and it's a great tool. I would like to know if it is possible to send an e-mail to all my customers.In fact, If I understand all customers are, by default, in the group "users". So I used the "Admin notification" and I selected the group "users" and checked "Also send to the customers in groups". All agents/admins received the e-mail but not my customers. Is it normal ? Have you any suggestion ? Thanks in advance.Regards, Cheupi