If I understand your question, you want somebody@yourdomain.com to have their mail sent to somebody@theirisp.com. If this is right, then just edit /etc/mail/virtusertable and add a line like this:
somebody@mydomain.com somebody@theirisp.com
You can also have all mail for mydomain.com sent to anotherdomain.com using the same format. The first entry is an incoming address or domain and the second entry on the same line is an address or domain the mail is sent to.