I have postfix running and directing mail to a smarthost at my ISP. I would like email with a particular outgoing email address to be forwarded to another server, specifically GMail; all the rest through the ISP. e.g. Outgoing mail labelled From: X@gmail.com should be sent via the X account on GMail, rather than through the ISP. This is fairly straightforward on end-user mail clients, so hopefully I just missed the obvious for postfix. Only one such address need be supported for this.