We recently signed up for Barracuda's Email Security Service which acts as a gateway for filtered mail.  I would like to see an outgoing gateway option on a per domain level.. For now, we are using the priority "hack" to flag all traffic from a specific domain to use Barracuda's outgoing gateway.  This is not a great option for a few reasons - mainly since Burracuda actually assigns a separate outgoing host per domain you set up on their side.