I am wondering how to set up so mail is delivered using external mx for mails generated locally on zimbra server where this domain exists.

I have a customer (example.com) who used to use our zimbra mail service, but have now changed their MX and main mail server to another service provider. But they still want to use our server as an mail archive.

The problem is when any other domain on this server tries to send email to this domain it is delivered locally instead of using their new server.

Any ideas ?