From the ISPConfig 3 Manual:
"188.8.131.52 Relay Recipients
If you have created email transports under Email > Email Accounts > Email Routing, you must also create Relay Recipients so that the server knows that it should accept those emails before routing them to another server. If you have created a route for a single email address, you must create a relay recipient for that email address. If you have create a route for a whole domain, and you know all existing email accounts of that domain, it is recommended to create relay recipients for all these email addresses; if you don't know all the email addresses of a domain, or there are simply too many, you can create a relay recipient for the whole domain, but you should keep in mind that the destination server can become a source of backscatter in this case because if a mail is sent to a non-existing address of the domain, the forwarding server will route it to its destination server, and because the destination server doesn't know that email address, it might send a bounce."