Clusterization would be one thing...how about the simple option to have plesk acting like a mail relay? I mean it's not hard to configure postfix that way, the point here is, it would need a option in the control panel so let's say disable local mail for one domain and enter a destination ip where the mail should be relayed AFTER being checked for spam and viruses. That way Plesk could be used as a allround Product. Actually we are having customers which just want pure virus/spam checking trough a gateway and have it relayed to their mailserver, we need a extra server for this right now.