[sieve] Restrict redirects to own domain only

Hello list,

I'd like to let my colleagues redirect mail automatically (via a sieve filter) to other mailboxes within the same domain, but deny redirects to outside domains (gmail, yahoo etc.) since this could lead to blacklisting (forwarding spam for example).

If I set sieve_max_redirect to 0 then no redirect is allowed, at all. I wish to let redirects to internal mailboxes still possible. Ideas ?

Yassine.