Forwarding Mail

We have a few outside sales reps who want to have there email forwarded to a different email address. We want joe@ourdomain to have his mail forwarded to joesmith@hisdomain. We tried setting up a mail rule that would forward any incoming mail to the new address, but sometimes an email does not get forwarded. We thought we would try using the forwarding address on the person document. That does not work either. People sending emails to joe@ourdomain get an error message stating they are not allowed to send mail through our Domino server. We can not allow relaying, so anyone know what we can do to fix this?