Forward email by recipient filter

I'm migrating my mail system from a server to another (different product).

I have to migrate all emails from one to the other. 

Behind the servers there is the fortimail in gateway mode. It works fine on my domain example.com. 

I need to forward mail sended to a selected recipient (john@example.com I have already migrated) from old mail server to new mail server because this is his new server after migration.

I cannot find a policy, a rule to perform this activity.

Regards,

Graziano.