we have a Basic hosting plan with lunarpages.
we have only one MX record for our domain.
we want to have at least two, and we've come to an agreement with some of our friends which are hosted in a completely different part of the world.
they do mail forwarding for our domain, and we will list their server with a secondary MX preference in our domain's DNS records.
but they want the same in exchange.

Q: how can we allow mail forwarding for someone else's domain on our server?