If customer request for all outgoing email forwarding, we do allow and only allow to forward to non-free email account. Incoming forwarding can use the Directadmin -> Email forwarding function
1) vi /etc/system_filter.exim
2) Scroll to bottom before version history and add the code below
if $sender_address_domain is userdomain.com
unseen deliver [email protected]
Replace userdomain.com with new domain and [email protected] with new email user.