[sieve] Restrict redirects to only own domain

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 mailboxes (gmail, yahoo etc.) since this is considered bad practice and 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.