Delivery Routing allows you to specify where an email should be passed to after it has been processed by Mimecast. It is required to specify one for your internal domains. External domains will route to their MX records when no policy is set.
Definition
The Delivery Routes definition specifies the address of where to deliver emails when it is used. You must configure the Definition first.
Set the scope for the policy under Emails From and Emails To
Press Save & Exit
Only one Delivery Routing Policy will apply to an email. If you need to ensure a policy is picked, you should enable the Policy Override option within the policy.