Cannot send workflow email notifications to contacts belonging to other customer than customer's request

Client uses workflow email notification and wants to send notifications to contacts via custom fields containing these contacts.
Why some of them receive emails, but not all?

You can only send notifications to contacts that have the same customer than the request.
If the contact belongs to another customer, the email won't be sent.

There is a way to bypass this restriction on contacts:
create a custom trigger (or query that will be used in the workflow) that will insert the desired values in the email queue.
The mail service will then make its job and send the notifications.