Impossible sharing a file with an external collaborator

Hello. 
I am an administrator of my box account. I can share files with an external user I have a message saying “The invitations could not be sent to the following people, because external collaboration is limited either by your company or by their company: xxxx@gmail.com"
I have tried with several email addresses and it only works with adresses with our own domain name. 
Where could I configure this? 
Thanks in advance. 
Olivier